New method, one controller... working for XBOX and PLAYSTATION.
For PS5, install ps4 version if there are problems
Launch a zombie in multiplayer.
Wait to arrive on the map then go to the console settings.
Disconnect internet for 2/3 seconds, reconnect it and go back to the game.
A black screen and an error message will appear.
A friend must be in the zombie mode and invite you, join him.
Then he must start a private zombie game (on the same map)
Wait....and bim !!!! .. black screen .... error message .... it's all good!