Hey dude. Nice to see someone pushing the envelope. Good luck with the willow graft that’d be a first.
I did a bunch of grafts at one point using living plants - the method is similar in most ways but instead of fitting together two matching V’s it is more like two half V’s fit together. It’s detailed somewhere in the early days of my current journal I think. I make opposing slits about 60-70% of the way through the two stems then slide them together to interlock tightly.
In the early days I wrapped the joint with whatever was around-which worked ok- but eventually found some Teflon tape in the shop which worked best. Teflon tape is actually the same thing as Gore-Tex, so presumably it breathes slightly. Living plants are pretty easy to keep alive while they graft- so you may have better results when doing tricky things such as you are.
As LA said, they’ll join best where the tissues line up best. However, even if things aren’t perfectly lined up, they do naturally tend to want to join together. I had regular parallel stems that were in close proximity merge into one stem without any attempt at grafting them on my part. (Edit- that’s not entirely true I guess- the stems were close together because they were joined by a graft nearby. But what I mean is- they were just regular stems- not sliced, scraped, or treated in any way other than being squished together side by side)
I also found a tree in the forest once that had separated into twin trunks. The two trunks of the tree had then grown back together into a single perfectly unified trunk farther up, forming a sort of very large oval hole in the mid section. It seems to be a natural inclination for plants to do this stuff, even if its maybe a difficult process for them at times.
