Yes, the lead developer of Shadow Complex entertained the idea of designing a co-op Metroid/Castlevania kind of game. But...
One would think that with all the ambitions that Donald Mustard and his team at Epic-owned Chair Entertainment are trying to accomplish with their XBLA love letter to Super Metroid — 3D graphics, a huge world, giant bosses, etc — that they might even have been thinking of multiplayer.
Multiplayer hasn't been done much in Metroid/Castelvania-style games. None of the side-scrolling Metroids have it. Castlevania games on the DS have dabbled with putting co-op in boss rush modes and time attacks.
Mustard loves the idea of trying co-op as a main mechanic in a Metroidvania kind of game. But he said it was an idea that just didn't make sense to prioritize for Shadow Complex. He imagines that, if he could do it, he'd want players to be able to solve complex puzzles, in concert even if they were located on opposite ends of the game's map. In fact, he said that that would be cool if a Metroidvania-style puzzle required players to be spread that far apart.
I suggested that the Shadow Complex developers could prototype the idea and offer it as a downloadable expansion to their game.
Mustard switched into "If we're fortunate to do more games like this"-mode.
If anyone out there wants another experiment in co-op Metroidvania, you better hope he's fortunate.
NOTE: Reader Blue Toast Likes Parenthises notes that this "sounds like Contra" more than it does a Metroidvania. Mustard talked to me about that being a key challenge. Contra can do co-op because it is linear. Making a Metroidvania co-op would have to allow two players to explore and backtrack, requiring much more complex game design. Good luck to anyone who tries it!