Inosculation is how two separate trees merge

Inosculation occurs when two trees begin growing separately, but come into contact with one another at some point during their growth. As the trees bump into each other, the damaged areas may eventually fuse and heal as a unit.

These types of morphed trees are often known as "marriage trees". I love the way they look. I've definitely stumbled across marriage trees before on walks, but never knew how they came to be. 

There are many people who intentionally cause inoculation between trees  as part of their art or landscaping or for the artform known as arborsculpture. There are some fantastic images of arborsculpture works here.

The images below come via Wikipedia's article about the phenomenon.

Photo: Roger Griffith (public domain)

The John Wesley tree. Photo: Alex Cameron (CC BY-SA 2.0)