Have you confirmed the policy direclty with US Airways? Always a good idea. The last time we connected through MIA to Brazil on two separate tickes, we took AA from DCA to MIA, then TAM on to Brazil, and AA was able to check our bags through to Brazil, but we had to get our TAM boarding passes at the TAM check-in counter at MIA, which I believe you will have to do, too, no matter what happens with your bags. You took risks buying two separate tickets, so now you have to make the best of it. Your other problem, of course, will be that with two separately-purchased tickets, you'll likely be under domestic flight baggage allowances for RDU-MIA. Earlier discussion on who sets the limits seems irrelevant.