Here is my Prototype 1 with all completed challenges. I duplicated the Truck and changed the texture for Player 2. Camera change key is Q for Player 1 and "Delete" for Player 2.
https://connect.unity.com/mg/other/prototype-1-24
Also, I completed Challenge 1 and added speed control (Horizontal axis) for the airplane.
https://connect.unity.com/mg/other/challenge-1-18