Quick and Easy Questions (please read the OP)

[Amaterasu]

I remember the opposite from what Crocodile says (but I was told in the CBox, so I can't link you to anything): a fused form can only have one Power-Up active at a time, because it's one "person". That power-up can be from either person, of course.

Also, the SP distribution is dependant on what the players agree on! One player could supply all the SP for the Fusion and Power-Up if they so desired, even if the Power-Up that the fusion uses is from another character. But if you want definite answers on either you'll probably have to ask Alex or Greggo.