I do this often and have never got any sort of letter.

I have had long term stays where I left in the middle.
I have checked in for a week, but left wife and kids to stay while I went off for a few days on business.
Even days where I had two reservation because I need a room for meeting but staying across town.
I have even used Platinum override to get a room and overridden a concurrent reservation.


I can't see any reason for Marriott to complain.
I am paying for two rooms who cares.
There are lots of reasons why this happens.

The only issue is if you are not really the one using both rooms.