That is amazing! I've never seen that before. It's called Inosculation and is pretty rare, well, I think it's rare. I've looked at many trees in my life and have never seen it. https://en.wikipedia.org/wiki/Inosculation
Inosculation is a natural phenomenon in which trunks, branches or roots of two trees grow together. It is biologically very similar to grafting. They sometimes self-graft and grow together as they expand in diameter. Inosculation customarily results when tree limbs are braided or pleached. I have this on my property with Willow and Bamboo ,,,I understand this is more common with some types ..the reason I knew this was I also was curious years ago when my grandson asked after noticing it on our property..