I'm afraid your approach won't work as is. You essentially need to use LODs over blended data, and that's not possible.
I found a similar question asked some time ago, and the recommended solution is to use table calculations: LOD from multiple Sources?
Please take a look and see if a similar approach would help in your situation. Let us know if you have any concrete questions about that.
Looks like I cannot use look ups (dimensions) across data sources. This issue is fixed in tableau 10 using cross joins. But thanks for the input Stoyko.