Short answer, probably look like the same person, but different because it's a character design so you have to tweak it somehow. Make them look more intense. At least from what we've all seen fusions always have a dual layered voice. If they have epic tits then they'll have epic tits. Goten fusing with Shihori didn't take away from the feminine attributes at all. Just carried over dark hair and a tom boy appearance. Shihori's chesticles were completely intact....huh...if there were saiyan twins who reached SSJ3, and then they fused...would they just look exactly like they did separately, but as one being...and would they have a singular voice rather than plural? Stupid olsen twins putting stupid ideas in my head...
(Also would the boobage be cumulative, since essentially they're the same person genetically merging with themselves, or would it still average out? WE MUST KNOW!)