“Jack” is rather ho-hum. “Jill” is nothing but a yawn. But “Jack & Jill” are magic… admittedly, tragic magic but that’s a whole other post.
The pairing becomes a character, an entity all its own. It exists separate and apart from its halves. It becomes more than its pieces. So, in my opinion, a successful romance novel will always have 3 leads: Character A, Character B and Character AB.
Bottom line: The story’s not done until you can count to 3.