Ben 10 isn't a show known for having fusion aside from merging mechanics a particular omnitrix had; So my two alien boys accidentally become one because of using a teleporter together, their bodies are so easy to break apart and put back together that combining just... happens.

It helps in that moment, but afterwards they have to go on a detour adventure to find a way to reverse this situation; but after they're finally separated, it kinda feels weird being apart. So an acquaintance builds a handheld device to let them become one at will; y'know... Just incase they REALLY have to.

You can certainly use your imagination beyond this point.