Capcom is really pushing the four-player co-op capabilities of Lost Planet 2, but it's not your only option. At Comic-Con last month, producer Jun Takeuchi and director Kenji Oguro told me about the amount of customization available for gamers, including the ability to disable co-op entirely.
If you choose to, Lost Planet 2 will let you disable the co-op partners and tackle Lost Planet 2 solo. It's not the developers' recommended path, as the game is designed with three companions (even if they're A.I.-controlled), but for gamers seeking an extra challenge, the option is there.
I won't be picking it, though!