On both my PRG and Plat card I had loaded the $50 off $250 hotel stay as well as the $60 off $300 Hilton stay. I checked out of the Hilton Hawaiian Village this past Saturday with a $800+ bill. At checkout I had them split the charge between both cards. I received 4 emails confirming the use of my amex offers, charge is still pending but it looks like these should stack.