You can get a heading from two GPS but they have to be mounted a known distance apart usually along or across the long axis of the aircraft. We use a system on ships with a 2 metre baseline and gives us a heading accurate to better than 0.1°. If you had the antennae on your wing tips your baseline would be 10 or 14 metres apart and the accuracy would be way better. Both position fixes have similar errors due to satellite mistimings and atmospheric errors but if the bearing is calculated between the two antennae then that is extremely accurate. Unfortunately no one makes the units for GA aircraft.