Assuming you have just the smart trainer and no secondary on-bike powermeter, the easiest way to do this is to use a Windows 10 laptop with ANT+ for one program and Apple TV for the other. Bluetooth is very "one-to-one" and doesn't like to share it's connectivity over multiple software.
Something like...
Windows 10 laptop with TrainerRoad and an ANT+ dongle (Bluetooth disabled, just to be sure)
- Connect power / trainer control / cadence over the ANT+ signal
Apple TV with Zwift (or RGT, FulGaz, Rouvy, whatever)
- Connect to power and cadence
- Leave the trainer resistance control not connected
You'll get all your power data and full ERG resistance control via TrainerRoad, while Zwift will receive your power data to move you along the course but not control the resistance changes.
It's a way of completing TrainerRoad workouts while still logging your Zwift "miles" and earning drops, etc.
NOTE: You can technically run both TrainerRoad and Zwift on the one Windows 10 laptop, but you'll need two ANT+ dongles plugged in, one for each program. Using ANT+ and Bluetooth on the one laptop at the same time via two training apps sometimes works, but often causes communication glitches.