A friend and I started co-op campaign last night right when we unlocked the ability to play online. He was able to join my session and we did a bunch of races together. Between loading screens, I saw a message or tip that said any co-op campaign will also be applied to single player campaign.
If that is true, what happens if he joins my campaign later after I played a few races and unlocked new venues? Does this overwriting his single player campaign by using my further progressed campaign? I ask as I don't either of us to miss out on content or lock any achievements.