CNC
Making simple multiplayer maps - how to make a simple multiplayer map, from 2-8 players
The biggest difference between multiplayer/skirmish and single player maps maps is that you need to specify the starting positions of the players. In a single player map, instead, you show where the camera starts, and the player can use whatever units he/she starts out with.
There is a video here that will show you how to set up a simple two player map.