I would assume that the system that works as noted above would also say that if you have one person booked in a room you cannot make reservations for more than one person.
Yes, you could. As long as there isn't a concurrent ADR, the number of people on the room reservation does not matter at all.
But if not, suppose a group of six staying in two rooms wants to have three go to CRT and three go to Prime Time at the same time on the same day. Plus, they want to bring Grandma, who lives in Orlando, along to CRT. Should they not be able to add Grandma to that reservation? Under the proposed system they'd have four at CRT and three at Prime Time when they can't book concurrent reservations for more than six. So Grandma's out.
Poor Grandma is not out. In this scenario...you just would need two different people to make the ADRs. Anyway, situations like this are too rare IMO to be considered when devising the framework for the system. So for those who have these rare types of exceptions (and those who choose to 'abuse' the system with actual double-booking), it will require an extra effort, but system will still benefit the overwhelming majority.