To fuse you both must know the fusion dance or someone obtains the potara earings.When you fuse your power level doubles.When Super Saiyans fuse they automatically appear as Super Saiyan after fusion.When you fuse only 1 person gets to controll it.When you fuse it is not permanate(unless you use the potara earrings), the fusion dance lasts for 1/2 hour, then you wait an hour before you can do it again.Also the new char gets the attacks of both of the characters.The Fusion Dance takes 5 days to learn.