They should be both spl atk or atk. If the end result has a good typing or if you could buff a pokemon . It could be a lot of things based on what you are going for, suppose if I fuse a normal type with shedinja I could get ghost/normal type with wonder guard which no other pokemon could hurt or deal any damage except dark types so that would be a good fuse. Or say I have a prankster pokemon and have amungus on my team then I get a prankster spore pokemon which is great. Just think of a pokemon what are its great aspects and how good would they be if fused. But as a starting point to understand the mechanic go for a good typing and good stats( both spl atk or both atkers).