"Professor Portal" achievement in Portal 2
No, it's not ok, I tried several times. And the time I did get the achievement was when I added the guy to friends list before I started the game with him.
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I previously stated that it was not necessary to be friends with your co-op partner. This is however false information.
EDIT: You get the achievement at the end of the track when the confetti falls from the ceiling. If your co-op buddy is in your friend-list by that time you will get the achievement. So it's all good to add your co-op partner as a friend during the course.