I apologize, these are all terrible, but I just have to get them out there:
Rouge the Bat v. Batasha
Astro Boy v. Astro Man
Geese Howard v. (Untitled) Goose
Cloud v. Mallow
Master Chief v. Master Hand v. Master Higgins v. Ken Masters
Raiden (MGS) v. Raiden (MK)
a rabbid v. a minion
Siegfried (Soul Calibur) v. Roy (Fire Emblem)
Vivi v. V v. VVVVVV
Sahelanthropus v. Sahasrahla (I know this makes no sense, I just want to hear Jaffe say it!)