Ne pas pouvoir être au four et au moulin

Literally: Not to be able to be at the oven and the mill

Example: “Je ne peux pas surveiller les enfants et cuisiner en même temps, je ne peux pas être au four et au moulin !” — I can't watch the kids and cook at the same time; I can't be in two places at once!

English equivalent: You can't be in two places at once

Origin: Under feudal banalités, tenants were obliged to use the lord's oven and the lord's mill and to pay for both. The two errands were the standard pair of obligations, which is why they became the standard image of an impossible demand.