In a nearby park there is an ash tree that has an evergreen tree of some species that is growing within the main crotch of the tree. Is this a rare occurrence, or does something like this happen often? What is likely to happen if left alone?
I found a few of them over the years. None were very big
unless the seedlings roots grew into a decayed part of seeded itself in a crack or union filled with lots of composted leaves the seedling will likely dry up at some point.
I found a huge oak on my property with a cavity low, near the ground, a bit like a tall cave. Inside, another tree had sprouted. I can't imagine it getting very big without rainwater and plenty of light.
I've seen many trees with another species that grew so close that the larger grew around it, but not completely surrounding it.
It is quite common in the strangler fig, Ficus aurea. If they strangle a real tree, it often kills the tree, because the tree cannot grow by adding growth rings. If they grow around a palm 'tree", however, the palm will often continue to grow, since they do not add growth rings. There are other species of strangler fig in other parts of the world, but that is the species I was familiar with in tropical South Florida.