You can only take control of 1 team at a time, so you would be the NE Coach(which I created B. Belichick) and you could adjust the depth chart etc. And if your still under that coach when you start the game, it will automatically put your controller on that team.
So once you start the game, it's set. You'd be in control of the Patriots and the CPU would be the Steelers.
Essentially everything works the same as a solo CFM, except you can toggle from 1 coach to another and take control of any team in the 32 team CFM.
So no, you wouldn't have to play the game again using the Steelers, once your games over it does everything just like a single team CFM would.
I'm just picking the team that won there game on Sunday irl and trying to win it for them in my CFM, easier said than done though.
I've lost 2 in row that were wins irl for those teams.