I have seen the trees with up to five varieties, but always of the same fruit. We tried one with five types of cherries, but whether it was poor soil, or poor grafting, only two of the five did much of anything. Were it not for foraging bears, the tree might have done a little better.
I really like the idea of this. A lot of people don't have space for more than one or two trees. Even with the same fruit - apples for example - having different varieties with different ripening times would be a big benefit.