Hi Doddins,
There is a famous site with trident maple trunk fusing (
http://www.dugzbonsai.com/tridenttrunk.htm). I have tried to plant a few plants together, and let their trunks fuse. What I have noticed is that these "groups" generally grow very weak (stay thin) and don't fuse. The same plants, when put in the ground with enough separation thicken much faster (there was a very interesting thread about competition of plants in proximity in this somewhere in this forum). This is why I gave up the fusing thing, although others appear to have made it work for ficus as well (
http://www.interfaceengineer.com/bo...jects/ficus.htm); I am sure other species work.
Maybe the trick to getting the trunk fusion to work is to have a configuration that minimizes the competition between the trees, a species that doesn't care much about growing in clumps (maybe like maples and ficus), and taking care of them and having the patience to replace dead seedlings when they die, etc.
You might want to do an experiment, and plant four of them together in a clump, and another on its own, and see what happens.
Good luck with your project.