Mix it up, perhaps? Have a few more switches happen, one girl winds up with no eyes, the other with no mouth, but normal-ish enough to pass on the streets at night.

Then the sapphire "runs out of juice." The girls panic, and go back to the store, and get told it has to recharge. And they're stuck for a while, but it recharges after 12 hours or a day or two. And then it won't put both girls back together at the same time, so they're stuck with it. :D