Unfortunately, I don’t think you can connect more then one device to the Dexcom transmitter via the iPhone app. So it is either xDrip+ on Android, or Dexcom app on iPhone. I chose Dexcom app on iPhone since it can be support offline communication with Loop, which I prioritize.
I am still interested in using the standalone watch though as the transmitter collector for long runs/exercise. I will test this out in the next few days. However, currently, I just use xDrip+ with “Nightscout Follower” as the hardware source. Using this prevents duplicate entries in Nightscout, since Loop already sends all treatments and BGs to Nightscout. To prevent duplicate entries, you’ll need to disable UPLOAD to Nightscout in the xDrip+ app.
See the end of this thread related to using xDrip concurrently with iPhone Dexcom app: