He did mention that he doesn't know why no-one has made an internet connected multiplayer version... Man, that would be awesome!
Co-op is clear, as you described.
Competitive: you each have your own lemmings: blue-green vs. green-blue. You have your own start point and end point on the same map. You can only control your own lemmings. Your goal-point will accept any lemming. Hilarious chaos ensues.