It is under Settings / Activity Profiles. Just create a profile for each bike and select that profile before your rides. The 820 saves the totals for each profile.
I have been searching for this when I replaced the 510 with this 820. The 510 had Bike profiles and Activity profiles.
You can also assign different bikes set in the "Gears" section from Garmin Connect (be it mobile or browser based). That's what I actually do. I have a bike (and the components for which I want to track usage) set as the default for Cycling activities and when I use a different bike I manually update the activity. I don't think you can assign different sets of "gears" to different Activity Profiles, but I might be wrong,.
