Contrary to all the answers on here, it is in fact possible to reset your progress in co-op.
Make sure you are the host (you invited your friend, rather than you were invited by your friend).
Start a co-op game.
Make sure you are in the hub when doing this.
Warning:
This applies to both players.
Make sure the other player is fine with you doing this.
Unfortunately, this doesn't work with ss_map .
You have to be in a session with another player, and this will set both players' progress.
Enable the developer console in the options,
then press ~ to open it.
If you are having trouble, see this question .
Then, enter the following command in the console: mp_mark_all_maps_incomplete
You might see a weird message about DLC, but the command will have no other visible effect.
Now, enter this command: changelevel mp_coop_lobby_2
This will reload the lobby, and your progress will be reset at zero.
You can use this command to change your progress manually:
mp_mark_course_complete <course>
Whereas the number is the number of the course you want to unlock (have finished), minus 1.
So to unlock course 1, use mp_mark_course_complete 0 , to unlock 2, mp_mark_course_complete 1 , and so on.
This means that mp_mark_course_complete 0 unlocks course 1 fully, and will then open 2.
There is also mp_mark_all_maps_complete if you want to get Professor Portal.
Thanks to fail badp♦ for allowing me to waste his time.
gaming.stackexchange.com/users/None/
I am wondering precisely why you would care to fully reset things.
I don't think you can , but it shouldn't hurt the experience at all if you don't reset it.
I will assume that you really just want to play through the intro puzzles with your friend.
If so, here are a few things to note: When at the level selection Hub, you can bring up the Menu and go back to the Calibration area.
This is the first area you start at when a new person starts playing, and has some simple puzzles that get you in the swing of cooperation.
When at the level selection Hub, you can look at the door above each puzzle set.
You will see little Orange/Blue rectangles.
These indicate how far each player has progressed through the section.
If you find a door with more of one color than another, you've found a puzzle set that you (or your friend) hasn't completed.
As you enter a puzzle area in the Hub, you will see a small monitor on your left that indicates, once again, how far each player has progressed.
There are small buttons under this monitor to select which level you would like to play.
gaming.stackexchange.com/users/32/Mag Roader
I know this is an old question, but I was just skimming through and had a bit of an addendum to the best answer here (the one involving the mp_mark_all_maps_complete command and all that).
At first I submitted it as an edit to that answer, but I think it's enough to warrant it's own post here.
So here it is.
Step-by-step how to reset your own progress (without affecting anybody else).
Launch Portal 2.
Select
"Play Cooperative Mode" Select "Standard Coop"
From here, open the Console (default ~ , otherwise check the related questions for more info.
CONSOLE COMMAND: ss_map lobby_2 CONSOLE COMMAND: mp_mark_all_courses_incomplete
Leave the map, and go play with your friends, as if you started from scratch.
This is especially useful for people looking to experience the transitions and extra few GLaDOS lines.
gaming.stackexchange.com/users/24650/Mikel_S
As the game is saved to the Steam Cloud and no option is provided to reset this, this is impossible.
The only difference would be the animations in The Hub for loading new zones, in order for the player to see them they have to play with someone that hasn't completed them.
Apart from that, you can just run to the game and selecting the first chamber in each zone...
You can, however, try to contact the Steam Support and ask them to reset it for you.
gaming.stackexchange.com/users/998/Tamara Wijsman
The whole point why I wanted this is to let my second Cooperative Testing Partner in Science™ (who got me Professor Portal, by the way)
experience the 5 unlocking sequences; I missed the first three myself.
Listening to GLaDOS is a large part of the Portal 2 experience.
Apparently, however, we can't just do that, so here goes Youtube scavenging. :(
Surprise surprise, finding decent videos of those sequences is challenging.
Let's plays drown out GLADoS ruining the whole point of this exercise, while walkthroughs have probably have everything unlocked already (when they don't skip the lobby entirely), so finding good Youtube videos of this is challenging.
So I'm marking CW to let you help me find those videos. ? ?
gaming.stackexchange.com/users/23/badp
Multiplayer progress in Portal 2 is saved to Steam Cloud:  Is progress in the coop campaign in Portal 2 saved?  How do you reset your progress to zero and experience the game from scratch with someone who has not played it yet?
How do you reset saved multiplayer progress in Portal 2?
gaming.stackexchange.com
<portal-2>
gaming.stackexchange.com/questions/20459
gaming.stackexchange.com/users/4298/Calvin Fisher
Start a co-op game in the hub. You have to be in session with another player and this will set both players' progress. Following some direct instructions can be of assistance. Furthermore, contacting Steam Support and asking for their help is also an option.
Users provide guided instructions based on the games buttons.  Contacting Steam Support and asking for help.