My answer is, that would not be a good idea, and likely would not work, they could rub and injury each other and one will hold overtake the other, two dwarfs would be a better idea, or you probably could graft "winter banana" apple onto the Apple tree and then graft quince on that, it should work, kind of as an interstem, Clara Fris pear is an easy grafting variety that is able to graft onto and should work as an interstem as well..