I've had to reserve two rooms for quite some time now with adult children up until this Christmas trip, I have also purchased two dining plans because I could not use credits for my family registered in the other room. In my experience, I have seen a multitude of things happen. Some examples were: CM wait staff only needing to see one band and charging all five TS on the plan even though the band said 2 or 3; CM wait staff responding to my comments about having so many left over TS credits by asking if I'd like to purchase meals for anyone else in the restaurant; a family being given a hard time (meaning that a manager had to be called over) at TREX to use more TS credits than the band said - he let them; Raglan Road CM being very specific about seeing two bands to total five TS credits.
So in my long winded examples, the point I have found is that (1) it has always been up to the restaurant to decide whether I can use my TS credits for non dining plan members and (2) it is the restaurants that are on the dining plan but outside of disney control (so to speak) that were the strictest. Given that...I never wanted to deal with it .... so I bought plans for each room.
This year I have decided to get the Deluxe plan on the room with two (2) adults and I will use my TS credits for all five of us. Even though they haven't spelled it out that I can do that....I feel more confident than I have in other trips that I will have no problem. Just my two cents of experience.