Because otherwise they would be both bleeding and screaming in pain…
-> The horn has nerve endings on the inside (meaning it has also blood vessels), as otherwise touching it would not interrupt the concentration of a levitation spell [like how Sweetie Belle did it once with Rarity].
Unicorn genetics. -> If one of them was male [X+Y], both of the “fusions” will be either male or herm.
Okay, makes sense. But why do they both have horns?
Because otherwise they would be both bleeding and screaming in pain…
-> The horn has nerve endings on the inside (meaning it has also blood vessels), as otherwise touching it would not interrupt the concentration of a levitation spell [like how Sweetie Belle did it once with Rarity].