Facesitting Challenge 5

Is this slave worth to become a human chair for these beautiful mistresses? Nikki has an idea to find out! They are testing his limits . They sit on his face 1 by 1 and stay there as long as they wish.. If he moves out, he fails.. But after each failure he gets a few faceslaps as a punishment. But since these slaps help him becoming a better chair, he has to compensate it with reward..