Are you sticking with your teammates? As a mccree player, I find myself more game changing by strangling and fading into the background and making the enemy paranoid to use flank paths and sometimes sneaking up behind them and coming out of no where with a high noon. Only engage tanks when my team is struggling with them or if they are not leading the pack.
A coordinated double McCree team doesn't need to stay on the flanks to be effective, especially if they've got Lucio and a tank of their own. And it's not so much that it's impossible to fight against as it's just been obnoxiously pervasive. Around half of my matches have been running double McCree so far today. Sometimes I just wanna play as Roadhog or something and not worry about being completely melted in seconds, y'know?