Who would they be, and what game type would they make?
I would combine Vigil with Platinum Games for a action packed, Zelda-ish type of game. Vigil made Darksiders, Platinum made Okami. They would make an awesome Zelda type game together.
Or Ubisoft Montreal with Edios Montreal to make a sci-fi futuristic open world conspiracy/espionage fps game. Ubi=FC3, Edios made Dues Ex HR. Perfect combo for great gameplay and great story.