In addition to what craigthemif mentioned: You can only get points on two rooms if they're billed to the same folio per the Honors T&Cs -- technically if you have two overlapping reservations you only get any credit for whichever one posts first which can (could?*) make certain scenarios tricky-- like if you're checking out of a property on one day and have an early flight into the next city for the following day -- one may be tempted to book an early night at the 2nd property to be sure you can get to a room on arrival -- but if you do this nothing will post for that stay [though the Diamond Desk has extended a courtesy on a couple occasions]

I would guess if you're seeing night credit for overlapping stays that's a coincidence (like nights rolled over from last year or one of the promos that gave double night credit) rather than actually getting credit for both reservations.

(*- With my current point balance and having Lifetime Diamond I'm not sure that I care as much at this point...)