Still can't help you for phone app, but it seems like some of the Garmin devices newer than my Edge 500 could select what bike they were on based on the sensor ID's it connected to at startup. If you didn't have a speed sensor that was unique to each bike, then you couldn't do it.

I haven't heard it discussed in years, but maybe some others know. A phone app certainly does get Bluetooth and some ant+ info about what it's connecting to, so they should be able to do similar. Whether they do..... I don't know.