you can get stock grafted apple trees too, two different varieties on the same trunk, it`s a grafting trick, growers can hybridise oranges with lemons to grow larger oranges, they are that closely related, you can graft the two stems to grow off the same rootstock, thats why you can`t grow a decent fruiting apple tree from a pip, if you look at the base of a apple tree you can see the "kink" where the stem is grafted onto a different rootstock to make it more vigorous, most fruit trees are done this way, mixing on the same rootstock was popular a few years back as a novelty tree for the garden, normaly it was one green and one red apple stem, I suppose abroad where citrous can be grown outdoors they had oranges and lemons on the same rootstock (google fruit tree rootstock)
[EDIT] linky thing explaining apple rootstock, same for citrous fruits...
http://www.gardening.cornell.edu/factsheets/ecogardening/appleroot.html
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