I am afraid the additional information you provide doesn't make much sense to me now. In your original post, it appeared you were asking about changing what is usually referred to as "shift" (i.e., some transmitters encode with positive shift, some use negative) and had just misused the the term "polarity" to describe that difference. It appeared you wanted to use a transmitter with one shift for the student, which would be connected to another transmitter with the other shift for the instructor, which might be possible depending on the specific designs. Remember, the trainer cord bypasses the RF section of one of the transmitters. Many, if not almost all, of the newer transmitters and receivers can be set up for either shift. HOWEVER, the shift had nothing to do with the trainer cord per se. The cord is simply a passive device for interconnecting the control signals from one transmitter box to another, and so long as the plugs and pin-outs were correct, the cord worked. I have never seen a cord that reverses the shift, but that is not to say there isn't one somewhere. Your new post still uses "polarity" in a way that makes me think you may actually be thinking of changing DC polarity, particularly when you refer to what is usually a passive device that can change the "polarity."
At this point, you need to refer to the manufacturer, manuals, or someone who has actually dealt with your specific system. Good luck.