If you’re going to do this, don’t use seedlings. Instead, start a lot of cuttings from one tree, and once they get big enough, try to fuse them.
I have seen a couple of the trident fusions that were on a form. The fusing pretty much worked, the trunk was really lumpy and fissured. Not my thing, but it was interesting. The reL issue was they started with seedlings. Each tree had slightly different leaves. Different shapes, different shades of green, there were different bark characteristics, some had tight internodes, some long internodes, some leaves were small, some large...
You get the idea. It didn’t look like a tree. It looked like a science experiment because of all the variation.
If they had all been clones from the same tree, the variation in the leaves wouldn’t have been an issue. And it would have looked orders of magnitude better.