If you do separate ressies for each room it can be done with each having their own credits.
Also, you may be able to do all or nothing. This would be where the other party would not have any credits on their room key card. I think that could be possible, but I would not be a happy camper with you if you did this to me.
If you are not separated from one another, you can control what's going on anyway.
I think you can explain in simple terms to this person that they have so many snack credits, etc. for their stay. Also, each time a credit is used it will be reflected on the receipts. You could tell if you pay very good attention who is using what with the credits. Tell this person they must keep all their receipts and you could do a bookeeping of the credits each day to keep score. You can tell by the places you go to, as well. I'm not sure about snacks, but I think guest services in your resort can give you a printout of where the credits are used and when. I know it's in their system. They can at least tell you if they won't print it out.
Calling Disney in this instance is probably still your best bet, if you really insist on splitting credits with one reservation. I doubt that you can do it, but they will have the best answer.