Headst/single portable Push to talk

Hey guys,  I have a 415cd with a sigtronics intercom and two portable push to talk buttons.  The issue seems simple but only one headset side tone voice can be heard even though both headsets work fine when not plugged into the ptt.  Radio transmission works fine and is clearly audible when transmitting.  While both headsets are plugged into the ptt’s only one headset voice activation is audible.  I’ve tried telecom, and David Clark ptts with no joy.  Switching the ptts and headsets makes no difference, still only voice on one unless a ptt is unplugged.  A bit convoluted but if anyone has a suggestion, let me know.  Thanks