After hours of trying to get this. Going through so many guides and deleting so many characters.
I stumbled upon one very obvious but unmentioned thing.
It has to be done with sync shot. Both you and the other person must then shoot within a few seconds. I popped it on both my account and the afk account I was using so you don't have to shoot just after then to get the trophy, It can be gained by both of shoot at the exact same time.
But again USE SYNC SHOT.