I've recently got a Garmin Edge 25 that I switch between bikes, and I've discovered that the Garmin Connect app lets you define "gear" - in my case, 5 different bikes. You can then select what "gear" you used for the particular activity, and it seems to be tracking the mileage on each bike accordingly. After each ride, I select which bike I used (there's one as default, but it's easy to change).

Might be a workaround for what you want to accomplish.