I have this fantasy actually.

-slave A has to give oral to slave B
-the slave giving oral must make the other girl cum as quick as possible
-the slave receiving oral must try not to cum at all
-they have a time limit of 20 minutes

-for every minute the girl giving oral takes to make the other girl cum, she gets 1 stroke of the cane
-for every minute the girl receiving oral doesn't cum, she get's 1 stroke less than 20

so if slave A makes slave B cum in 15 minutes then slave A gets 15 strokes and slave B gets 5