The following method allows you to play singleplayer maps with a friend in coop mode.
It takes advantage of the commentary system to modify maps in order to make the playable in coop without actually modifying any map files.
Moreover, only the host is required to have the necessary files since they will sync through the net.
Here's what you'll need:
This zip contains the "fixed" commentary files for 36/64 singleplayer maps.
I'll fix more maps in the future but for now this should do it.
To use them, just extract the contents of the zip in your portal2\maps folder, host a coop game and when in the lobby open the console, type "commentary 1" and then "changelevel <map>" (replace <map> with a singleplayer level of your choice).
Later on I'll edit the lobby so that you can choose singleplayer maps from there.
Sometimes you might get stuck, just enable cheats and use noclip.
The host will not be able to move until the other player connects, don't worry.
If you notice that both players can't move then it means that the level is not fixed yet.
After you've hosted a singleplayer map, if you try to create a coop session, your partner will disconnect with a "#Valve_Hidden_Game" error. Just restart the game and it should be alright.
Hope you enjoy it!