Sitecore 7.2 Update-2 (rev.140526) was recently released, and one of the entries in the Release Notes was this:-
- Layouts and renderings
- The Select the Associated Content dialog did not handle multiple data sources correctly. It only used the first data source and ignored any additional data sources. This has been fixed. (389483)
I tested out his solution back when the post was written, and it worked perfectly. However I found that it was broken in both Sitecore 7.0 and 7.1.