You will need a second Windows Phone device for this. Multiplayer is only available locally, meaning you will need to connect two devices over a Wi-Fi connection. Make sure both of your phones are capable of running the game; you will need 1gb of RAM to play the game. You can use your gamertag on both devices simultaneously, so won't need to purchase the game again.
Pick King's Speedway, 1 lap. Once the race has started, quit out with one of the devices and finish the race with the other. Rince and repeat ten times.