For now, you can only add one vehicle in QuickBooks Self-Employed. The program will calculate the vehicle deduction using the standard method (total business mileage). Regardless how many vehicles were used, the amount will be the same.
You will also need to manually record the miles driven by the other person. Just go to the Miles menu and click Add trip. Then, record all other details including the Distance before clicking Save.
Please see these articles that talks about recording miles in QuickBooks Self-Employed: