The problem is that you are providing the same connection string key for different models and this error occurs because Open Access caches the metadata after the first OpenAccessContext is created. And after that if you use the context for objects which are not contained in the cashed metadata, the exception would be thrown.
There are two possible solutions:
1. Use different connection strings for any model you have. When you create new RLINQ file using the Add Domain Model Wizard, you should add a new connection string to the web.config/app.config for every new model.
2. Use AggregateMetadataSource
to merge the metadata of two different context instances that use the same connection string. It can be implemented following this guide
I hope that helps. If any other questions arise, do not hesitate to contact us.
the Telerik team
Q3'12 SP1 of OpenAccess ORM packs Multi-Table Entities mapping support. Check it out.