People have begun to warm up to games with peripherals; Rock Band is the uber-example of this. The problem has become getting more of the needed peripherals to make the game work. Extra guitars for both Rock Band and Guitar Hero 3 have yet to hit the market, for instance, causing some gamers to go to dishonorable lengths to get extra equipment.
This problem has also been found in the PS3 title Time Crisis 4. So far there is no way to get a second gun to play two-player games. Namco-Bandai's response is simple: you should just buy the game again.