When you are practicing mask removal, do not "prepare" yourself for it. Have your buddy do it at random times.
Hmm...I really don't think I'd advocate surprise mask removals, especially if your buddy is not a trained instructor (and even then). This is not the type of environment I'd enjoy diving in, and there's real potential for problems that don't seem worth the benefit of the exercise.
If you really have to do it this way, I'd suggest having your buddy tap your mask as a signal for you to remove the mask yourself, but not have him do it in some kind of unexpected ambush.