Nature offers many curiosities, and an attentive observer will certainly have encountered one in the form of two or even more trees that have grown together at their trunks or branches over time.
In principle, trees of the same species can grow together. The fused branches form when they are very close to each other or overlap. The friction between the two branches damages the bark and exposes the so-called cambium, which is responsible for growth. When the cambium cells of the two branches touch, the tissue fuses: the two branches connect and from then on share the water and nutrient pathways.
But trees of different species can also grow together, although in this case, it would be more accurate to speak of a "mutually enclosing" process: Since they contain different genetic material, the two branches cannot "fuse" and subsequently form wood that simply grows around the other branch. The two trees remain independent of each other because there is no exchange of nutrients.