I did all of insane co op(couldnt do it myself) yet i did not get the achievement for completeing al acts as dom or marcus in co op.
Thing that sucks is from what i can tell, you cant see which ones you completed in insane and which ones you didnt.
any ideas anyone?

Start from the beginning and work your way up until you get the achievement.

If you missed a chapter in a1 for example you wouldn't be spending too much time.

Have fun

lol
atleast i can run through it on casual with a buddy
i just did half way through act 3 - the end of act 5 on insane with my friend.
So i know its gotta be within the first 3 acts somewhere lol

Seriously have fun.

If you don't get it by friday, let me know and I'll jump on and help

same thing happened to me so I just did it all again on casual, doesnt take to long but its usually one of the ones in the beginning

try doing the training thing again with carmine in the beginning

this is pretty much what happened to me and my friend, we went through it, he was getting the hardcore achievement and i was getting normal (already had casual) we went through every act in the game, every chapter, and it says were missing one chapter for the co-op achievement, but we both got our difficulty achievement