My brother has played Portal 2 on my account and got a decent chunk of the achievements without specifically trying for them. Shamefully, I haven't yet played it, but I want to of course. I really enjoyed getting all the achievements in the first game and want to try for them in the second, but I want the regular Steam alerts to show up when I get one so that I know for sure that I got it. Naturally, this won't happen for any achievements that my brother already got.
Thus my question: Can I clear Steam achievements (set them all back to unachieved), so that I'll get alerts and be able to track them properly? I know I can clear the game progress that's saved via Steam Cloud but I don't want to do that since it would destroy my brother's progress, and I know there are some issues syncing the achievements between the game and Steam anyways.