Noggin
I question your response. What's you evidence?
Error in GPS is unlikely to be common to all 24 satellites. You only need 4. You typically can pick up 6, 7, 8 upwards. You'll appreciate that gives many calculation permutations.
Even if all 3 GPS were identical they may still lock onto to different satellites and thus not share the same error.
If they are different, which is what I said, then you have the benefit of different averaging algorithms to iron out the effects of error.
My GPS tracks 8 satellites but only in rotation. I'm sure more expensive ones can track several simultaneously. No doubt that produces differences in the calculations.
So what error (apart from interference/jammimg) do you think GPS is affected by that can cause you a problem on two different receivers, assuming you are not down to the minimum 4 satellites?
Incidentally my £100 GPS has an RMI which visually shows passing a waypoint just like an ADF, and it automatically moves to the next waypoint as another visual cue.