You can make your SELECT statements access 2 different sources. I can't remember the exact syntax but I have seen it posted here on UT before. essentially you do something like the following:

SELECT Field1, Field2 FROM [ODBCSource1]![Table1]
UNION (SELECT Field1, Field2 FROM [ODBCSource2]![Table2]);

Obviously my syntax is wrong but I hope this shows you that you can actually do it. I know you can include the provider strings within the actuial SQL statements.