There is a connectIQ data field for Garmin devices that allows you to dual record multiple power meters to the same fit file. (ANT+ Power Meter by takura87 -
https://apps.garmin.com/en-US/apps/7...a-23f029a75e3b)
I use this to record both a P2M NGeco and Assioma pedals when I ride outdoors. The Assiomas are connected to the Garmin the normal way, and I put the ANT+ ID of the NGeco in the settings of the ANT+ Power Meter ConnectIQ app via Garmin Express. Then you have to add the data field to a data screen in your Outdoor Cycling activity profile so the Garmin knows to look for the extra power meter.
The data for the second PM is recorded in a developer field in the fit file, and I use either DCRainmaker's Analysis tool or Golden Cheetah to see the info if I want to compare the two PM's.