A separate TeslaFi.com account is required for each vehicle. The logged vehicle for each account can be selected in settings->account->vehicles.
Two TeslaFi.com accounts can be linked together from settings->account->vehicle->link. Once linked, and new menu option will appear titled 'Vehicles' from the menu bar where you can easily switch between accounts.
If you would like to share tagged locations from one account with another account an option is available when linking the accounts. If would like to share tagged locations from vehicle A with vehicle B you would log into vehicle B and link it with vehicle A while choosing the option to share tagged locations. After this vehicle B will use the tagged locations from vehicle A's account.
Customer support service by UserEcho