Crappiehnt,
It won’t matter which unit the waypoint was marked on as the coordinates for the GPS Receiver that the unit was using will be saved (or the location of the waypoint if you used the cursor to mark a waypoint in one of the sonar views). The coordinates would get shared to the second unit much like what you may do with a friend whom you swapped waypoints with – it is just done automatically is all. Since these coordinates are based on your position on the earth, they can be referenced with any other GPS Receiver as they are all reference from the satellites.