Apparently after the progress bar completed it provided them a tool to decrypt the code on the other login (dratmann / h0nee).
It linked to an image that hinted (very faintly) at the possibility of co-op gameplay:
https://bayimg.com/image/oalehaaca.jpg
From here: https://www.facepunch.com/showthread.php?t=904084
By the way, they have already created a forum section for Portal 2 on the Steam forums
.
EDIT: Other people think the “co-op” actually refers to the ARG, as in the cooperation shown by people to solve it. More images showing “Co-op Initiative Completed” or something of the sort were found, backing that up.

