You could be really sadistic and punish the other when one misbehaves That's sort of a mental predicament because the actions of one causes pain in the other.

I am assuming you have thought of the obvious... 2 female slaves? Lesbian floor show! Maybe including a challenge which must be completed (best behaved slave, one that pleases you the most, slave that adopts the best position, slave that finds the toys you have hidden somewhere in the room first...) to determine which slave gets to wear the strap on

Here's a thought... make them dependent on each other. Remove half the senses from each of them. A combination of blindfolds, gags, ear plugs and bondage so that each is dependent on the other for some of thier ability. Then you set them a task. This can be made as easy or hard as you like. For example, if you have one blindfolded and the other sighted but has hands bound then the one who is sighted has to tell the other what to do. However, if you gag the one who is sighted or put ear plugs in the one who is blindfolded... Tasks could be something like trying to clean the room or do other housework, giving you sexual pleasure, reading and memorising a document or whatever you can think of. I wouldn't, by the way, expect them to achieve this task until they have had a hell of a lot of practise working together...