You can create Individual Profiles for your different bikes on your Karoo.
Here are the steps to create/edit a Bike Profile:
Open the "Settings" app from the Home Screen.
Tap on "Bikes".
Tap on the profile that you want to edit.
Select "Edit" and rename the profile as you like.
Tap on "2105.00 mm" under "Wheel Circumference" profile.
Select the value by referring to this article: How-to: Set Wheel Circumference
You can add as many Bike Profiles as you prefer.
Karoo - Total distance from Activity app
Bikes - Total distance covered on all the bikes (after updating to 1.54.624.3)
You will now be able to select a Bike in addition to Profile and Route while starting a ride.