Cross database queries are often one of those facts of development. LINQ to SQL offers several advantages that you might want in your project. Unfortunately, a Query cannot contain references to items defined on different data contexts.
Here’s one solution.
Modify the Source attribute of the tables that resides in a database other than the one mentioned in your connection string. For example, change "dbo.tbName" to "otherDataBaseName.dbo.tbName".
Keep in mind the security context of your connection; you will need a single user that can connect to both databases (the databases will also need to reside on the same server).
Tip: I used a second dbml file for adding the tables that are hosted in the second database. Then I copied the dbml over to the original dbml file and modified the Source attribute.
Mudnug 