Nope, you can blend different data sources together. Excel against SQL Server or SQL Server against Oracle, or Oracle against Access and Excel and SQL Server will all work just fine.
Maybe you're thinking about doing a SQL-based UNION of data in which one "Table One" from SQL Server is essentially "dropped right on top" of a table with duplicate structure but different data - "Table Two"? In this approach the data sources are generally the same (but there are workarounds here, too - if you're sneaky).
Just try it, I think you should be OK
Russell is correct. From time-to-time I find myself blending Excel or Access data with tables coming from Teradata or DB2. The only caveat I've seen with blending is when working with cubes - you cannot blend cubes for a cube data source needs to always be primary, and when blending there has to be a primary and secondary.
I 'll point you to a nice white paper on this subject.
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