I found a way to play workshop coop levels split screen.
First you will need to fully download the map. Check if the map is in C:\Program Files (x86)\Steam\steamapps\common\portal 2\portal2\maps\workshop
You will see a load of folders, just check in each one for the map you just downloaded. For example I downloaded a map called "Mad coop - part 1" but the folder that it was in was called "900972024285156860".
Go inside the folder and look or a BSP file (it may have a portal 2 logo on it, and the file will be named in numbers e.g. "1345153644". Copy the file and paste it in "C:\Program Files (x86)\Steam\steamapps\common\portal 2\portal2\maps".
Rename the file that you just copied to the actual name e.g. "mp_coop_mad_coop_part_2"
Run the game and just type in "ss_map mp_coop_mad_coop_part_2". The map should load fine and work correctly.