I think what
JDiver is saying is that if you use a Hilton AX (I think) CC to pay, you get x points/$ - and if that CC is registered to AA idine, you get y points/$. So you can earn both Hilton and AA!
Likewise, if you have a CO MasterCard (that earns 1 mile/$) and have it registered to UA idine, you'll esentially earn both CO and UA miles for your dine!