I would agree that you should factor in the additional adult costs at AoA, then see if you can get two rooms at another value for close to the same cost. Sometimes you can go through a third party, like Expedia or Orbitz, and they will be matching the general public discounts for Disney resorts and have codes you can apply on top for additional discounts. I just think it would be more comfortable for everyone to have more space.
However, every family is different. Only you know if your family works well in one hotel room (and a kind of smallish one at that). If you have traveled before and know that you don't have any issues in one room, then go for it. I don't do well for more than a night or two sharing a hotel room with more than one other person. If we're on a road trip, I can handle it for the overnights along the way, but when we reach our destination and are planning to stay put for a few days, I generally get my own room. But I have friends who pretty much all stay in one room wherever they go and however long they stay, because they would rather spend any additional money on other things than the room. So there really isn't one right or wrong answer. Do what works best for you.