Zubax gnss 2.2 on apm 3.6.9

Hi I just installed 2 zubax gnss 2.2 on a black cube with arducopter 3.6.9
Gps receiption looks great , but I have questions regarding the on board magnetometers
1- The Apm on boot recognizes 3 magnetometers (2 internal and 1 external uavcan )
Is there a way to choose which magnetometers to use ? (example 1 internal and the other 2 external uavcan ?)
2-If I will fly with the actual configuration ( 2 internal and 1 uavcan external) How can I assure that the external one used will be always one of the 2 gnss modules ? I am afraid that the system will boot up and start to use the external mag from the other unit , which was not calibrated…
3-Can I deactivate the on board mag from UAVCAN GUI on one of the 2 units in order to avoid this? , if yes how to do this ?