Using moving baseline heading and canbus magnetometer as backup not working

My setup uses two Ardusimple GPS units set to allow the moving baseline system to determine heading. I would also like to use the Here2 GPS unit purely for the magnetometer as a back up (ek3_Mag_Cal set to 6). With that fitted using the CANBUS the magnetometer does not appear to be sending any data (canbus has been enabled in the params and also the switch internal to the Here2 GPS is set to CAN). I have a constant bad health report and the heading of the aircraft is always due north. I have: Compass_Enable set to 1, Compass_External3 is set to 1 and Compass_use3 is also set to 1. I have disabled the internal compasses. If I disable all the compasses then the heading is spot on. Any ideas as to why I can’t seem to get a reliable heading? Many thanks.

I think the features you are looking for are only implemented in master. Are you using that version?