Splitscreen for PC

[Führer]

I recently leanred that Portal 2 can play splitscreen on pc. And i dont mean the through the console command. Its a new feature. I can't seem to find any info on it at all. How do i start the splitscreen. It says in Cooperative Menu, press X on the second controller. What does that mean? Can i use Mouse+Keyboard and xbox controller or do i have to have 2 M+K or do i need 2 contollers? I'm really interested in this and really want it to work. ( i accidentaly posted this in workshop discussion)