Have you figured this out? I have the same issue, I've even tried connecting both the Edge and the watch to the same chest strap and taking my watch with me in my cycling jersey pocket when going for a ride so it still picks up the HR data, but it doesn't work
Changing the GCM Device Setting » Activity Tracking » Preferred Activity Tracker should help, though it is a bit inconvenient having to do it manally each time you swap devices. Turning on Physio TrueUp on both devices (if they are both compatible with that feature), may perhaps take care of it automatically, but I cannot test it for the cofirmation, since I do no use multiple devices with the feature.
Thanks for the reply! Physio TrueUp is always turned on for both devices and the Edge doesn't have the option of preferred activity tracker.