Fruit salad trees, or even as some call them "Franken-Trees" are not some exotic product that Alice brought back through the looking glass nor are they a futuristic GMO . They are simply multiple fruit tree buds/branches grafted onto a single root stock to produce functional and aesthetic trees that produce multiple varieties of fruit and spectral light shows at blossom time.
One artistic and enterprising man, Syracuse University art professor Sam Van Aken has created a Franken-Tree he calls the "Tree of 40 Fruit". Van Akens tree is comprised of 40 different branches from various stone fruits. Plums, Peaches, Apricots, Cherries, Nectarines and even almonds of assorted cultivars are grafted together.
Aken uses a technique known as "Chip Grafting ", the buds of one fruit tree are cut off and grafted onto the branches branches of a rootstock tree. Other grafting methods such as Whip, Cleft and Saddle Grafts can also be used. See - Grafting Fruit Trees
Branches from the assorted fruit trees will grow off of the root-stock of a related tree. The rootstock tree should be indigenous and well acclimated to the area in which it is planted, but the grafted branches it hosts do not necessarily have to be.
When well executed with the correct cultivars, a franken-tree can be continuously in bloom all spring and early summer with a rainbow of appealing blossoms. They can also be producing fruits for a good portion of summer and early autumn.
Although professor Van Akens work has been with stone fruits, other families of fruit can also be made into franken-trees. Fruit salad trees / Franken Trees have been successfully made from multiple varieties of apples and citrus as well.
"Stone fruit salad trees grow peaches, plums, nectarines, apricots and peachcots. Citrus salad trees offer a winter and summer orange, mandarins, lemons, limes, grapefruits, tangelos and pomelos. Multi-apple trees boast between two and four different kinds of apples and multi-nashi trees produce between two and four different kinds of Asian pears." The Science of Pomato Plants and Fruit Salad Trees
If you'd like to try your hand at creating your own Fruit Salad Tree, it is not as difficult as it seems, just master some simple grafting techniques. Don't expect to create a tree such as Professor Akens right away, his were made over multiple seasons. There are also some companies that will sell ready made fruit salad tress. One online resource is the Fruit Salad Tree Company which markets "6 DIFFERENT TYPES OF FRUIT all on the ONE TREE. " in either Stone Fruits, Citrus, Apples or Asian pears. Another is Groworganic.com which offers 3 in 1 grafted trees.
Some examples of similar grafting done with Herbaceous [non woody ] plants include the Tomtato - Pomato Plants in which Potatoes and Tomatoes are grafted together. The Egg N Chips Plant which is basically the same thing done with eggplants and potatoes.