You can fuse trunks of willow to create one fat one, though I don't know about adding new ones at future times... a bit odd. I would very carefully scrape the top layer til you JUSTexpose some green (not a fat top to bottom deep slice), but be very careful about how you bind them. Honey? I wouldn't think so... never heard of it in bonsai or any other plant graft. But why cut and paste if your doing the other thing above? Are there no branches at all? Certainly you can take off inner branches that are in the way of the trunk line, but... am I missing something else?

This post was edited by moochinka on Thu, Aug 14, 14 at 21:35

The usual method is to secure multiple seedlings together. There is no need for cutting or grafting techniques. Small greenwood seedlings are usually used.They are secured tightly and will grow together because there is nowhere for them to go but together. Problems are seedling that die. After all you are trying to grow multiple trees at once. Also some seedling will try to dominate, shoot up higher than the others.. it would probably be advisable to keep some extra seedling in case of interior seedlings failing.
Should be a nice project. Good luck and tell us how it progresses.