the way I would do this is launch a game that you want in split screen mode, then press TAB and adjust the settings until it is in that mode. It's sticky from that point on; every time you launch that game, it'll show up in that mode.
No muss, no fuss with lists, command line switches or anything else. Yeah, it'll take a bit of time to set all the games you want in this mode, but that''s the type of work that requires a beer and friend, so it's not so bad.