The Rock of Prometheus v1.5
Install in:

Steam\steamapps\common\portal 2\portal2\maps

You and your partner must both have the map installed.

Go into a server together (like the hub world).

enable the developer console(enable in keyboard/options then push ' ):

Whoever is playing Atlas (the host) must then enter in the developer console 

Changelevel mp_coop_rock_of_prometheus

Enjoy!
~ magicrat 2011