[To be fair that isn't entirely different from how things are for John in his own universe. It's not like he really understands everything about his powers there either, so this is one more item on his to-do list. Still, he holds up a hand and begins ticking items off with his fingers, holding one finger up for each point.]

There can only be two items. I cannot fuse more than two at a time. There doesn't seem to be a size limit, but I also haven't tried fusing anything really big yet. I can fuse things that already have other magical properties. There will always be hints of what the items used to be, even if I cannot always control what they look like at the end. Those are about the only rules I've figured out so far. I haven't fused much besides the brain lamp and some jerky water.