The AccessPORT for the MS3 and the MS6 are the same and can be used on both cars, naturally. However, one AccessPORT will not flash both cars. Since your friend has installed his AccessPORT on his car, it will only work on his car. You cannot use the AP to work on your car too while it is still "married" to your friends car.
Simply put, you can only use one AccessPORT on one car at a time. You cannot flash multiple cars with a single AccessPORT. That is, you can't go in on an AP unit with your buddies and flash all of your cars with the same unit.
Travis
COBB Tuning