I am also curious about this. My guess is that if the skill has been designed well, then the same modifiers that both players are using with mark of impurity do not stack. If each player has different modifiers with their MoI however, then both players' modifiers are combined into one ultraMoI when both players cast their MoI on the same enemy. That would be good for teamwork and creating an organized two-player setup. It would mean that a boss could have a MoI applied to it that has more than 10 points worth of modifiers affecting the boss.

I have no idea if that is how MoI functions or not, but i think it would be good for the game if that could be done.

curious if even devs know this ;)