Dear Joshua, you centered the problem, I tested everything and is exactly how you point out! I re-wrote the query according you suggestion, and it behaves as expected!
Thanks a lot! Also I have asked the same question here http://answers.semanticweb.com/questions/28143/filter-query-variables-across-two-named-graphs-in-the-same-dataset-jena If you have an account there you can copy paste your answer, or I can refer to your answer on this mailing list if I knew what that is. I wish to close this a contribution for the many other practitioners.
