I don't really know how multiplayer would work. Racing wouldn't be the best because there aren't really 2 different routes on the maps so you wouldn't bump into eachother on the same wall, for example. If there were to be multiplayer I assume the other players wouldn't be able to collide with each other and it'd just be a time comparison.
Either that or he makes new maps specifically for the game. 3v3 capture the flag: parkour edition, would be fun.