I think it would be foolish to even consider moving the miles around. While you point out that using the UA miles for travel will gain you "nothing on AA in terms of accumulating mileage for premier status", actually, even if you used AA miles for travel you would not gain anything on AA for premier status.

I think you gain quite a bit by having miles in two programs that are at award levels - if capacity controls hamper your use of miles in one program you may be surprised to find that miles in the other program are an easy solution. I'd forget the magical tricks - if you don't have a full mile for a full mile, i know I would not even think of trading acounts around.