i debated with myself whether to put splitscreen into the xmas pack, cause right now i think my method can be improved. the curent method uses views, but my ideal method is to use surfaces (actually i want to use surfaces for drawing almost everything).
in addition to the help kain provided, make sure in the room settings that your room's view 1 and 2 "View in room" have the same width and height as your view 0's, and that the "Port on screen" has the same width, but have the height half of the main view's. also, make sure that view 2's "Port on screen" Y is half of the "View in room" height. and lastly, make the views follow whatever objects you want them to (player 1 should be linked to view 0 and 1 while player 2 is linked to view 2).
if you want separate huds then you'll have to make use of the view_visible in the draw event of the hud.