I use EclipseLink 1.1.4 on Tomcat 6. But I'd like to use EclipseLink 2 due to JPA 2 features. So I would appreciate any help how to configure EclipseLink 2 on Tomcat (what files and where to copy). And I'd like to know whether is possible to run EclipseLink 1 and EclipseLink 2 on one Tomcat server
For JPA 2.0 you need the JPA 2.0 version of the persistence.jar on your classpath.
Many features of JPA 2 are also available in EclipseLink even when using the JPA 1.0 persistence.jar, as the eclipselink.jar includes any of the new classes added in JPA 2.0.
I'm not certain on how class loaders work in Tomcat, be in thoery if you deployed the persistence.jar in your war as a library, then you should be able to have another application in the same server with another war with a different version of the persistence.jar in it.