If you have your own card......
Start the game with your card in and his card out. If you leave both in when you start the game, it will load whatever profile it finds first starting with the card in slot 1. If it does not find one in slot 1 it will read the card in slot 2. Leave his card out.
Start the game and create a profile, and leave that card in slot 1. Now whenever you turn it on, it will end up reading the profile in slot 1, which will be yours. You can then insert his card into slot 2.
As a bonus, when you earn enough money, you can buy cars from his garage.