1. Join or host a multiplayer server.
2. Find a vehicle preferable with gunner and commander seat, or create a mission with two playable units and a Gorgon.
3. Enter the vehicle in a passenger, driver or gunner seat.
4. Ask your buddy enter the vehicle in a higher ranking seat, say the commander seat.
5. Observe how you have the seat switching options in the scroll down menu before he enters, and its gone after he entered the vehicle.
6. Ask him to exit the vehicle and observe the option return.
I tested it with a friend for 40 minutes in the Gorgon, Strider HMG, Panther and T-100 and the same thing happened every time.