How to add another vehicle to your booking
In short, this is not possible.
A booking can only be linked to one vehicle. You have the ability to change a vehicle on a booking (Depending on the terms of the specific location) however, adding another vehicle to a booking is not possible. For more information on how to change a vehicle (if permitted) please check the following article.
In order to get around this, a separate booking needs to be created to ensure that you have paid for the other vehicle.