I hope he doesn't play both. Cincy is good enough especially if he's coming off a Wimbledon title as we hope!
I take back my previous thoughts on this issue.
If he wins Wimbledon and Rafa flames out early, then Roger will be trailing Rafa by just 500 points in the race and suddenly #1 ranking will be within reach. Hence, he will probably try to play both NA masters and full schedule (but not itsy bitsy tourneys) after that.
On the other hand, if he does not win Wimbledon, he won't be in the running for #1 and so it won't be important to play many tourneys. To give himself extra chances at winning another GS at USO, he might confine the warm-up tourney to just Cincy, in this case.