I didn’t get a resolution, WattBike support took months to reply and finally said that simultaneous dual connectivity like this is not supported.
Honestly I’m really frustrated and disappointed in both Zwift and Wattbike in terms of their support and fixing bugs. I’ve had a few bugs reported for over a year now and no ETA in sight of fixing them. "Free ride" sections of workouts don't disengage ERG mode on Wattbike Atom - #50 by Alex_Walton If anyone asks me if Wattbike is actually supported in Zwift I will tell them it’s not. Regret the purchase and looking for another platform.