I don't know why I did not think of this earlier, but some years back I purchased a second house that the previous owner had planted two plum trees in the same hole. One tree was a purple leafed tree, and the other a green leaf tree. They are now full grown plum trees, each growing into a half circle. Between the two trees they make one complete circle. They both have grown to the same height and spread, and seem to be healthy trees and produce fruit . Maybe two citrus trees varieties with the same growth characteristics would do the same. - Millet