Most comp marketing offers stipulate that you need a break of 2-4 days between offers (I know that Venetian/Palazzo does this). Or, they say that there is a limit of the number of continuous comp days (Harrah's has been known to do this). I've had success with back to back @ Harrah's, but I knew I was breaking the rules, and was prepared just in case.

Probably three ways to approach :

(a) Full disclosure, call marketing and just ask or advise you would like to book a second offer (after your first).

(b) Just try to book the second offer, hoping that nobody connects the fact that you are already at the property (or one of the sister properties) --- and then, claim innocence if questioned at check in

(c) Use a host (preferably one who you have a relationship with) to book the additional offer, basically allowing for the "exception" if one is applicable in this scenario.