The presence of a second player in any mode is accomplished through split-screen, which reduces WWII Aces' already simple graphics to those of a slightly high-resolution PSOne game. This doesn't prevent spotting any objectives, though - all targets are denoted by red triangles, making them easy to spot even at great distances.
WWII Aces is a decent title that is meant to be played cooperatively. So long as you have another friend close by, the $40 shouldn't sting your wallet too much.
Apr 2, 2008