Both circuits were AFCI/GFI protected in the breaker panel.
Both circuit home runs ran to one junction box before splitting.
1st circuit was 20A single gang outlet, nothing else.
2nd circuit was 15A for direct wired radiant heater (line thermostat controlled) that would draw max 6A.
All grounds were connected in the junction box.
After consultation with the heater/thermostat company I learned that AFCI/GFI breaker cannot be used with the thermostat. It requires a conventional breaker. Conventional 15A breaker installed for the 2nd circuit.
circuit 1 trips immediately when circuit 2 breaker is switched on.
Circuit 1 does not trip with circuit 2 breaker off and vice versa. (Btw before switching from AFCI to conventional style 2nd breaker only worked if 1st breaker off and vice versa)
Heater (2nd circuit) works fine as long as other breaker is off.
Outlet (1st circuit) works fine as long as other breaker is off.
Eaton breaker panel and Eaton br115 and Eaton All-in-one 20A breakers.
I’m open to all suggestions to resolve.
Thanks very much