Currently, you could totally do that. One room could even have no plan at all! As long as someone with the dining plan is “paying” for the meals, they don’t care if the others at the table are part of the same reservation, or part of the dining plan at all. Sharing credits between more people than are paying for the dining plan is one of the biggest “loopholes” that can make the deluxe plan a great value.
It doesn’t sound relevant to your situation, but DxDP credits are different from the standard DDP’s TS credits in that they are also “pooled,” and not sorted out by age. So you can use a credit technically purchased for a child in the room on an adult’s meal. I most often see one room with DxDP and one with QSDP (where the credits are also pooled and don’t have to be used on kids meals) or nothing at all. But if you do like a lot of TS restaurants, having the second room on the DDP works as well.