I never did go into detail about the mechanics of how this works, did I? Well, it was still being polished at the time of the testers demo, so that may be why.
Originally, Soulfire weapons were offhand items posing as weapons. I'm pretty sure I coded in dual-wielding just before the public release. It certainly wasn't in the tester's demo, so maybe that's why it looks new. Either way, you were only able to form a weapon for one battle before Aeyr's prologue ended, so I felt it was worth sharing.
What IS new, without a doubt, is that I figured out how to lock in his Soulfire weapon to that you could change what's in his sword arm, where in the current release all of his equipment was completely locked down.