Interesting thanks for posting. I have come across a few specimens where one tree grows out of another. There are some particularly unusual examples of these ''double'' trees at a place on the Stiperstones in Shropshire called ''The Hollies'' which has some of the oldest Holly and Rowan trees in the country. Some of these ancient holly trees have rowan trees growing out of them, it is thought the rowan seeds in birds droppings have lodged in the holly trees and then fertilised and grown on. The Rowans have split open the Holly trees damaging them however both trees survive and now grow as one tree, a rowan and holly growing together as one tree. For those interested the history of these trees is quite interesting, the area called ''The Hollies'' is the remains of what was once a ''Hollins'', which is an ancient Holly tree forest where the holly trees were pollarded by the local lead miners in order to feed the holly leaves to their livestock during the winter months.
For those interested this link explains a bit more
http://www.telegraph.co.uk/topics/christmas/3566374/Hollins-One-of-Britains-last-ancient-holly-forests-is-saved.html