First a bit about the environment:
We use a program called Clearview to manage service relationships with our customers, including call center and field service work. In order to better support clients and our field technicians we also developed a web site to provide access to the service records in Clearview and reporting. Over time our need to customize the behavior and add new features led to more and more things being tied to this website and it's database.
At this point we're dealing with things like a Company being defined partly in the Clearview database and partly in the website database. For good measure we're also starting to tie the scripting for our phone system into the same website, which will require talking to the phone system's own database as well.
All of this is set up and working... BUT we don't have a good data layer to work with it all. We moved to Linq to SQL and now have two DBMLs that we can use, along with some custom classes I wrote before I'd ever heard of Linq, along with some of the old style ADO datasets. So yeah, basically things are a mess.
What I want is a data layer that provides a single front end for our applications, and on the back end manages everything into the correct database.
I had heard something about Entity Framework allowing classes to be built from multiple sources, but it turns out there can only be one database. So the question is, how could I proceed with this?
I'm currently thinking of getting the Linq To SQL classes all set for each database, then manually writing Linq compatible front ends that tie those together. Seems like a lot of work, and given Linq's limitations (such as not being able to refresh) I'm not sure it's a good idea.
Could I do something with Entity Framework that would turn out better? Should I look into another tool? Am I crazy?
The Entity Framework does give a certain measure of database independence, insofar as you can build an entity model from one database, and then connect it to a different database by using a different entity connect string. However, as you say, it's still just one database, and, moreover, it's limited to databases which support the Entity Framework. Many do, but not all of them. You could use multiple entity models within a single application in order to combine multiple databases using the Entity Framework. There is some information on this on the ADO.NET team blog. However, the Entity Framework support for doing this is, at best, in an early stage.
My approach to this problem is to abstract my use of the Entity Framework behind the Repository pattern. The most immediate benefit of this, for me, is to make unit testing very simple; instead of trying to mock my Entity model, I simply substitute a mock repository which returns IQueryables. But the same pattern is also really good for combining multiple data sources, or data sources for which there is no Entity Framework provider, such as a non-data-services-aware Web service.
So I'm not going to say, "Don't use the Entity Framework." I like it, and use it, myself. In view of recent news from Microsoft, I believe it is a better choice than LINQ to SQL. But it will not, by itself, solve the problem you describe. Use the Repository pattern.