Wondering the same thing. Have you figured it out?

Yes, but only with 2 controllers. Just load up the custom map from console and when in the game type ''connect_splitscreen localhost 2'' in console. That works for me.

@ThatHager for me that crashed to menu. Any other ideas?

@InfiniteBananaTheorem
http://forums.steampowered.com/forums/showthread.php?t=1029890
Try and see if you can't find the answer here, I think this is the one I looked at back in the days.

I have also been trying to find out a solution to this problem. Great mod but i want it to work with the custom campaigns.

To use connect_splitscreen localhost 2 you need to load the custom map with ss_map command from the menu.
To know how the custom map name you need to load it on SOLO Campaign, then open console and it is just below a line in green that says "Left 4 Dead 2", copy, go back to menu, load the map from console with ss_map command, then once in game use connect_splitscreen localhost 2 command, it works like a charm.

do you think you could do a video tutorial of that on youtube and then paste the link here? that would be very helpful. Thanks for the info, dont really understand it but i will try this later.

Heres your tutorial bro, well, actually I didnt explain anything because Im not english native speaker, so just pay close attention to the video and check out the description.

Edit: I added subtitles, be sure to turn them on.

https://youtu.be/FIURwV-_tUM

Thanks for the info, great help.

its getting
RejectConnection: loopback - #Valve_Reject_Server_Full
#Valve_Reject_Server_Full

wen i do this