Your question about driving your AP from multiple sources is actually quite common and quite easy to do and something we do for people quite frequently.
With the single level AP's that use only 1 wire for the RS-232 signal such as the TruTrak or Trio units, you simply need a Single pole, Double Throw (SPDT) switch that is On/On or On/Off/On. A good one to use is a miniature toggle as it barely takes up any room in the panel.
Here's what you do.
1) Run the RS-232 sense wire for the AP to the center terminal on the back of the switch.
2) Run a wire from each of your GPS's to the outside 2 terminals on the back of the switch....one wire for each unit to a specific terminal....not both units to both terminals.
That's it. When you select one position or the other, you'll be selecting either unit to drive the AP. If the switch is on/off/on the center position will be off, but with an On/On switch you'll only get 1 or the other unit driving the ap.
The switches are cheap and available many places. I think we sell them for a couple bucks, but you can get them at your local rat shack store as well.
Hope that helps.
Cheers,
Stein.