To use multiple data base as back end first u need to write table as databasename.dbo.table_name.
suppose u have 2 data base first one is test and second is online and u need to find records from both data base from two different table user and usermast.
simply u need to write.
Query:-
select a.*,b.* from test.dbo.user a inner join online.dbo.usermast b on a.employeeid=b.employeeid