To clarify: Anyone on the reservation can use credits. You do not all have to be together.
So if your MIL wanted to spend the day at Epcot while you guys went to MK, you could each use your KTTW cards to get meals using dining credits. As the PP said, just make sure you all know how the credits work.
My SIL took her family of 8 (6 kids, 2 adults) a few years ago and the older boys ate at ESPN every day for lunch. Well, they thought this only used a CS credit because it was lunch, not dinner. At the end of the trip, they had dozens of unused CS credits and quite a few TS meals charged to their credit card. This was a costly mistake, as they had paid for the dining plan up front. They would have been much better off to not get the dining plan, but nobody bothered to learn how it worked.