diff --git a/openjpa-lib/src/main/java/org/apache/openjpa/lib/conf/Configurations.java b/openjpa-lib/src/main/java/org/apache/openjpa/lib/conf/Configurations.java
index b3317a508..b256916ff 100644
--- a/openjpa-lib/src/main/java/org/apache/openjpa/lib/conf/Configurations.java
+++ b/openjpa-lib/src/main/java/org/apache/openjpa/lib/conf/Configurations.java
@@ -99,6 +99,41 @@ public class Configurations {
         return clsName + "(" + props + ")";
     }
 
+    /**
+     * Return a plugin string that combines the properties of the given plugin
+     * strings, where properties of <code>override</code> will override the
+     * same properties of <code>orig</code>.
+     */
+    public static String combinePlugins(String orig, String override) {
+        if (StringUtils.isEmpty(orig))
+            return override;
+        if (StringUtils.isEmpty(override))
+            return orig;
+
+        String origCls = getClassName(orig);
+        String overrideCls = getClassName(override);
+        String cls;
+        if (StringUtils.isEmpty(origCls))
+            cls = overrideCls;
+        else if (StringUtils.isEmpty(overrideCls))
+            cls = origCls;
+        else if (!origCls.equals(overrideCls))
+            return override; // completely different plugin
+        else
+            cls = origCls;
+
+        String origProps = getProperties(orig);
+        String overrideProps = getProperties(override);
+        if (StringUtils.isEmpty(origProps))
+            return getPlugin(cls, overrideProps);
+        if (StringUtils.isEmpty(overrideProps))
+            return getPlugin(cls, origProps);
+
+        Properties props = parseProperties(origProps);
+        props.putAll(parseProperties(overrideProps));
+        return getPlugin(cls, serializeProperties(props)); 
+    }
+
     /**
      * Create the instance with the given class name, using the given
      * class loader. No configuration of the instance is performed by
diff --git a/openjpa-lib/src/test/java/org/apache/openjpa/lib/conf/TestConfigurations.java b/openjpa-lib/src/test/java/org/apache/openjpa/lib/conf/TestConfigurations.java
index 43bd0e718..f67b40928 100644
--- a/openjpa-lib/src/test/java/org/apache/openjpa/lib/conf/TestConfigurations.java
+++ b/openjpa-lib/src/test/java/org/apache/openjpa/lib/conf/TestConfigurations.java
@@ -12,6 +12,9 @@
  */
 package org.apache.openjpa.lib.conf;
 
+import java.util.HashMap;
+import java.util.Map;
+
 import org.apache.openjpa.lib.test.AbstractTestCase;
 import org.apache.openjpa.lib.util.Options;
 
@@ -77,6 +80,62 @@ public class TestConfigurations extends AbstractTestCase {
         assertEquals("baz baz", opts.getProperty("biz"));
     }
 
+    public void testCombinePlugins() {
+        assertPluginsCombined("jpa", null, 
+            null, null,
+            "jpa", null);
+        assertPluginsCombined("jpa", null,
+            "jpa", null,
+            "jpa", null);
+        assertPluginsCombined("jdo", null,
+            "jpa", null,
+            "jpa", null);
+        assertPluginsCombined("jdo", new String[] { "foo", "bar" },
+            "jpa", null,
+            "jpa", null);
+        assertPluginsCombined("jdo", new String[] { "foo", "bar" },
+            "jpa", new String[] { "biz", "baz" },
+            "jpa", new String[] { "biz", "baz" }); 
+        assertPluginsCombined("jdo", new String[] { "foo", "bar" },
+            null, new String[] { "biz", "baz" },
+            "jdo", new String[] { "foo", "bar", "biz", "baz" }); 
+        assertPluginsCombined(null, new String[] { "foo", "bar" },
+            null, new String[] { "biz", "baz" },
+            null, new String[] { "foo", "bar", "biz", "baz" }); 
+        assertPluginsCombined(null, new String[] { "foo", "bar" },
+            "jpa", new String[] { "biz", "baz" },
+            "jpa", new String[] { "foo", "bar", "biz", "baz" }); 
+        assertPluginsCombined("jpa", new String[] { "foo", "bar" },
+            "jpa", new String[] { "biz", "baz" },
+            "jpa", new String[] { "foo", "bar", "biz", "baz" }); 
+        assertPluginsCombined("jpa", new String[] { "foo", "bar" },
+            "jpa", new String[] { "foo", "baz" },
+            "jpa", new String[] { "foo", "baz" }); 
+    }
+
+    private void assertPluginsCombined(String cls1, String[] props1,
+        String cls2, String[] props2, String expCls, String[] expProps) {
+        String plugin1 = Configurations.getPlugin(cls1, 
+            Configurations.serializeProperties(toProperties(props1)));
+        String plugin2 = Configurations.getPlugin(cls2, 
+            Configurations.serializeProperties(toProperties(props2)));
+
+        String res = Configurations.combinePlugins(plugin1, plugin2);
+        String resCls = Configurations.getClassName(res);
+        Map resProps = Configurations.parseProperties(Configurations.
+            getProperties(res));
+        assertEquals(expCls, resCls);
+        assertEquals(toProperties(expProps), resProps);
+    }
+
+    private static Map toProperties(String[] props) {
+        Map map = new HashMap();
+        if (props != null)
+            for (int i = 0; i < props.length; i++)
+                map.put(props[i], props[++i]);
+        return map;
+    }
+
     public static void main(String[] args) {
         main(TestConfigurations.class);
     }
diff --git a/openjpa-persistence/src/main/java/org/apache/openjpa/persistence/PersistenceProductDerivation.java b/openjpa-persistence/src/main/java/org/apache/openjpa/persistence/PersistenceProductDerivation.java
index 04d19de3d..ec67d85a2 100644
--- a/openjpa-persistence/src/main/java/org/apache/openjpa/persistence/PersistenceProductDerivation.java
+++ b/openjpa-persistence/src/main/java/org/apache/openjpa/persistence/PersistenceProductDerivation.java
@@ -32,10 +32,11 @@ import org.apache.openjpa.conf.OpenJPAConfiguration;
 import org.apache.openjpa.conf.OpenJPAConfigurationImpl;
 import org.apache.openjpa.conf.OpenJPAProductDerivation;
 import org.apache.openjpa.lib.conf.AbstractProductDerivation;
-import org.apache.openjpa.lib.conf.ProductDerivations;
 import org.apache.openjpa.lib.conf.Configuration;
 import org.apache.openjpa.lib.conf.ConfigurationProvider;
+import org.apache.openjpa.lib.conf.Configurations;
 import org.apache.openjpa.lib.conf.MapConfigurationProvider;
+import org.apache.openjpa.lib.conf.ProductDerivations;
 import org.apache.openjpa.lib.log.Log;
 import org.apache.openjpa.lib.meta.XMLMetaDataParser;
 import org.apache.openjpa.lib.util.Localizer;
@@ -382,10 +383,27 @@ public class PersistenceProductDerivation
 
         @Override
         public void setInto(Configuration conf) {
-            if (conf instanceof OpenJPAConfiguration)
-                ((OpenJPAConfiguration) conf).setSpecification(SPEC_JPA);
-            super.setInto(conf, null);
+            if (conf instanceof OpenJPAConfiguration) {
+                OpenJPAConfiguration oconf = (OpenJPAConfiguration) conf;
+                oconf.setSpecification(SPEC_JPA);
 
+                // we merge several persistence.xml elements into the 
+                // MetaDataFactory property implicitly.  if the user has a
+                // global openjpa.xml with this property set, its value will
+                // get overwritten by our implicit setting.  so instead, combine
+                // the global value with our settings
+                String orig = oconf.getMetaDataFactory();
+                if (!StringUtils.isEmpty(orig)) {
+                    String key = ProductDerivations.getConfigurationKey
+                        ("MetaDataFactory", getProperties());
+                    Object override = getProperties().get(key);
+                    if (override instanceof String)
+                        addProperty(key, Configurations.combinePlugins(orig, 
+                            (String) override));
+                }
+            }
+
+            super.setInto(conf, null);
             Log log = conf.getConfigurationLog();
             if (log.isTraceEnabled()) {
                 String src = (_source == null) ? "?" : _source;
diff --git a/openjpa-persistence/src/main/java/org/apache/openjpa/persistence/PersistenceUnitInfoImpl.java b/openjpa-persistence/src/main/java/org/apache/openjpa/persistence/PersistenceUnitInfoImpl.java
index 901eb01e8..5352a18a5 100644
--- a/openjpa-persistence/src/main/java/org/apache/openjpa/persistence/PersistenceUnitInfoImpl.java
+++ b/openjpa-persistence/src/main/java/org/apache/openjpa/persistence/PersistenceUnitInfoImpl.java
@@ -33,6 +33,7 @@ import javax.sql.DataSource;
 
 import org.apache.openjpa.lib.conf.Configuration;
 import org.apache.openjpa.lib.conf.Configurations;
+import org.apache.openjpa.lib.conf.ProductDerivations;
 import org.apache.openjpa.lib.meta.SourceTracker;
 import org.apache.openjpa.lib.util.Localizer;
 import org.apache.openjpa.lib.util.MultiClassLoader;
@@ -404,20 +405,14 @@ public class PersistenceUnitInfoImpl
             }
             metaFactoryProps.put("Resources", rsrcs.toString());
         }
+
+        // set persistent class locations as properties of metadata factory,
+        // combining them with any existing metadata factory props
         if (!metaFactoryProps.isEmpty()) {
-            // set persistent class locations as properties of metadata factory
-            String factory = (String) Configurations.getProperty
+            String key = ProductDerivations.getConfigurationKey
                 ("MetaDataFactory", map);
-            if (factory == null)
-                factory = Configurations.serializeProperties(metaFactoryProps);
-            else {
-                String clsName = Configurations.getClassName(factory);
-                metaFactoryProps.putAll(Configurations.parseProperties
-                    (Configurations.getProperties(factory)));
-                factory = Configurations.getPlugin(clsName,
-                    Configurations.serializeProperties(metaFactoryProps));
-            }
-            map.put("openjpa.MetaDataFactory", factory);
+            map.put(key, Configurations.combinePlugins((String) map.get(key),
+                Configurations.serializeProperties(metaFactoryProps)));
         }
         
         // always record provider name for product derivations to access