What is Coop 1v1
Coop 1v1, is a non-official feature of the game. Also knowned as "shared army" The goal of this feature is to allow multiple player to play with only 1 ACU, and to share the control of all unit.
This lead to different situation : for exemple we can have 2 players controling 1 army, and 3 players controling the other army. Be carefull the more player you get on one side, the more "cancel order" can be made. Leading to a lot of frustration. If the rating difference between 2 players on the same is too big, there is good chance that the good player would have played better alone. Which can lead to even more frustration.
How to play Coop 1v1
There is a mod that have been created. But that has issue for release.
The only way currently to play it is by allowing cheat. (put sandbox on if you want to be able to chat with your ally after deleting your ACU).
Choose which ACU will stay in game (and the spot where they will start) launch the game; then every player that weren't choose for keeping ACU need to alt-delete their ACU (same way as ctrl k, but remove explosion to not kill reclaim). Player that deleted their ACU have to alt-F2 (open cheat console), and double click the name of their ally that still have his ACU alive. then close console. Now every one can control every unit of their side.
Since it is needed (atm) to use cheat for playing this feature. Every one can kill the game by spawning unit, or looking in opponent view. You need to be fairplay.
You will often see people playing 20x20 maps on this feature. 10x10 maps can be player too. 5x5 map are too little to allow people micro/macro without canceling allies order. 40x40 would be a good size, if there wasn't a lag due to pathfinding. (40x40 would have been great for 3v3 on coop 1v1)
some TD want to host one tourney of this special feature. There is yet no tourney that have been hosted.