The cards for each room will have credits for the total number of nights X number of people on the reservation for that room. If you're there for 7 nights, and there are 3 people in the room, then each room key will have access to 21 TS, 21 CS and 21 snacks (I'm assuming you're on the basic plan).
The cards have the number of people in the room printed on them. Theoretically you're not supposed to use more than that number at a time with that card. So yes, in your case you should plan on using 2 cards (one from each room) when you all go out together.
I have had occasions where the CM took it all from one card - they asked first - because it was just easier for them and they could tell we were all together. Not supposed to work that way though.
First visit: Disneyland, July 17, 1955 (well, somebody had to be there on opening day!)
Nov 23 - Dec 2 AS Movies
Dec 2 - Dec 11 Boardwalk
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Disneyland - Labor Day 2007