On the KY97A, there is Sidetine pot located on the top of the case which you can access through a small hole. It should be clearly labeled "Sidetone".
You can adjust sidetone by turning either clockwise for more or CCW for less. (I think that is correct but trial and error will test that.) A little goes a long way on this adjustment. (Make note of where it is when you start adjusting so you can get it back where it was initially, if you need to. I don't believe that the pot will allow you more than one 360 degree turn end to end, though.)
Also, some intercoms automatically isolate the copilot when they are switched off, so you might check on that. It could also depend on how you have the jacks wired to the radio and the intercom.
I had the same concern on mine, but learned that my intercom isolated the copilot when it wasn't turned on.