Someone is interested to play a co-op using a software like Hamachi or EvolveHq for certain activities (like Mayhem or Trail Blazing)? Please, stated here and say which mods you have installed and if you have cheats activated.
Edit 2: If you want to make easy to find someone that would like to play co-op for this game too, leave your nickname for me that I will create a list for possible players:

List of GOG's user for co-op online:
Nickname on GOG / Nickname on Evolve / Use Mods? / Use Cheats?
DeadFishEye / OlhoDePeixe / No / No

Edit: I managed to make it work with another person from GOG using the software EvolveHQ! We played without mods. It's necessary to both of you have the same mods, or none at all, to play, or the game will crash.

To play it in co-op online with someone, donwload the EvolveHq client and registry an account. Now open the client and log in. The Evolve should detect your game. Now you need to create a party clicking in the Evolve red button (left side of the Evolve's window) and select "create party...". It will open a widow to configure a party. Select the blue button "Set Party Game", type Saints Row 2, click next, select to host a party and set the maximum number of players to a number and the minimum to a number equal or less than it.

Now that your party is ready, you need to invite a friend to it. Click in blue button "Send Party Invite" and typing your friend's nickname in Evolve or just send him the recruit link to do it.

Now open the game, which Evolve should recognize, and select, in the main menu of the game, the Co-op option (not the multiplayer), enter a desired name for you/your match and select to begin a new game or load one, then your friend just need to go to Co-op and select to find a game.

P.S.: if I wrote something wrong or did bad with the grammar, just say it.