Well... maybe "fuse" isn't the right word. Lagann has a drill that can break through other gunmen and allow it to gain access to the other gunmen's controls. So neither gunmen change, but Simon gets control of the one he's "fused" with. I'm not really sure what else to call it.

[He's still a little lost with the technical terms. That's Leeron's specialty, not his.]

It can't control more than one, no. But it did manage to control a massive um... "battleship", I think? It's a giant gunmen that's capable of fitting hundreds - if not thousands - of the normal sized gunmen in it. Lagann is much smaller than a normal gunmen, so it's really impressive.