This guide assumes you already know how to use dev console and play in split screen with working controls, there is plenty of guides for those.

Normally if you tried to connect to a Valve server using "connect_splitscreen" command, it would only connect one player. This is because it must go through a lobby before connecting to the actual server, thus losing the second player in the process.

But there is a way to bypass this.

First you need to bind the "connect_splitscreen localhost 2" command to an function key. It must be a function key or else it won't work in menus. I'll be using F8 in this guide.

Type in console: (include quotes)
bind f8 "connect_splitscreen localhost 2"



For Hosting:
Make a lobby like you normally would and when ready press the start button to find the server. Now as soon as you see the Campaign Poster press F8 on your keyboard, nothing might happen visually, but when you load in the game you should be playing in split screen.


For Joining:
As soon as you see the Campaign Poster when joining game, press F8 on your keyboard, it should load 2 players now. It won't work if server had only 1 player slot left.
EDIT: Above doesn't seem to work, see below for workaround.

For lobbies that haven't been started yet, as soon as you see loading poster, press F8, you should be now stuck in infinite loading, open console and type "connect_splitscreen localhost serveriphere:porthere 2" and now it should load. Server IP can be found at top of the green text spam.

For lobbies that are already started, as soon as you join a session, RAPIDLY press F8 until you see loading poster, you should be now stuck in infinite loading, open console and type "connect_splitscreen localhost serveriphere:porthere 2" and now it should load. Server IP can be found at top of the green text spam.

NOTE:
If you got red text in console that says "Sending connect packet to unexpected address iphere" then you were lucky enough to connect to the server with the split screen packet and the connect_splitscreen step isn't necessary.


Does NOT work on local servers.