It is possible to graft multiple varieties of fruit onto the one espalier fruit tree or to create fruit salad trees. Multi-graft trees can be a great use of space, can spread ripening out over a longer period of time and can help to overcome pollination issues.
When grafting multiple varieties together, it is important to consider the vigour of each variety. Whilst most of the fruit tree vigour will be determined by the rootstock, some of the vigour is determined by the scion. When you have a tree that consists of multiple varieties of mis-matched vigour then you are likely to end up with an unbalanced tree.
A classic example of this is a multi-graft apple with Granny Smith and Pink Lady on the same tree. The Pink Lady is a much more vigorous variety and will soon come to dominate the tree. In a few years your tree will become a Pink Lady Tree with a tiny Granny Smith branch at the bottom. Variety vigour must be matched on Multi-graft trees, or much time must be spent on pruning back the vigorous variety to reduce dominance.