It worries me. I don't know what notion of foot quality is being used, or how to measure it. Even once that has been agreed, it is surely possible that the quality of the left foot will turn out to be equal to that of the right; in which case the subject fails to denote, and we're in real trouble.
Keen as I am to improve the state of the world, I propose the following replacement. I think it captures the intent of the original, without being susceptible to that catastrophic mode of failure:
"At least one of your feet is such that the other is not better than it. Put forward such a foot!"
Be sure to agree in advance which quality metric you're using.