Perhaps this will be helpful: https://steamhost.cn/steamcommunity_com/app/926860/discussions/0/3051735738020461678/#c3052862273808453011

Thanks for such a quick response. Unfortunately that method hasn't worked for me despite numerous attempts.

I've just looked at the achievement picture again and had a thought; I've been using a controller for P1 & keyboard for P2 - does it need to be done on two controllers? I doubt it but thought I'd ask just in case.

Input method shouldn't matter. As long as you are in a campaign, one player wins and another player gets to cross the finish line, the achievement should trigger. The code hasn't been touched for years, so it is supposed to work.

Oh, and sometimes the achievements don't show up until you save and exit the game, something with steam client. Try fully exiting steam and restarting the game fresh just in case.

EDIT - DIDN'T WORK (full details at end of post)
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Original Post:
I've done this on many various sessions - about twice a day for the past week.

I only started the Steam client about an hour ago and am about to try again now. I'll do the race, save & quit out of the game normally then close Steam. I'll then re-open Steam (and BRS in case it needs the game open to pop) and let you know the result.

As mentioned, I've done this quite a few times already so am not expecting success but fingers crossed...!
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Edit:
Won first race 1-2 (I'm getting scarily good at steering using my chin!); controller for p1 & keyboard for p2. Nothing popped. Saved and exited the proper way (as I always do - barring one time the game crashed on me) & fully closed & exited Steam.

Restarted Steam and BRS, still nothing has popped.

I seriously doubt it but thought I'd state the build showing for me in case I somehow have an old version and it's not been updated for some reason. I'm just trying to think of anything possible as, from what you've said, there is no reason as to why this achievement hasn't popped for me.

v1.9.0
Build: 202304111718
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Edit#2:
Another thought (again, just trying to think of anything that might be the cause at this point): Is it possible that, when I got the 4 player achievement, that this should've popped then but was somehow overwritten/cancelled by the 4 player achievement popping? I did do the 4 player race in campaign (and won it) so it should've counted for the co-op achievement too, right?

Like I say, I'm just trying to think of any possibilities here. My apologies if nothing that I've mentioned has been even remotely helpful.

No problem at all spajus - I appreciate the fact that you're still checking the forums and are quick to help solve player problems

Thanks for the additional information; I'll try and finish a season tonight and will update afterwards - even if it's only to leave player confirmation for anyone else in the future who has the same question as me.

EDIT:
It popped! Didn't actually pop until I started the second season (after advancing after the cutscene) so was worried at first but it did so all's good :)