Yea you can do all kinds of fun things with trees..... They even do it commercially.... I saw one online nursery that sold "Fruit Salad" trees..... Basically they would put 4-5 different types of citrus trees and graft them all together so you could have a tree with oranges, Lemons, Grapefruit, limes etc....
With apple trees you can take one rootstock and graft more than one typs of apple scion to it to make a tree that will grow seveal different types of apples.....
The origins of say a Granny Smith Apple Tree or a Red delicious or anyother type of fruit tree, the way I understand it to be anyway, is that all of those trees are brought up by one single tree that exhibited a certain set Phenos ( Just like with MJ). that tree was clones and/or grafted and now there are orchards filled all ove the world witht he exact same tree with the exact same genetic make-up.... This is a good thing and a bad thing... You can get a consistant fruit, but then too you can get a strain related disease and wipeout the worlds supply of Granny Smith apples never to be regained...
Now you can Grow an apple tree from seed and it will grow..... It may even bare fruit, BUT and this is where it gets interesting.....it WONT be a granny smith.. It will be only a basic Field Apple not worth eating and not worth growing....