Please remember: when you're using 2 GPS connected to 1 aircraft system/gauge, you have to tell which GPS source is connected to the system/gauge.

This is the purpose of the Master Device setting.

Examples:

2 devices + 2 different gauges: master device irrelevant

GPS #1 Pilot Side            GPS #2 Copilot Side
      |                             |
      v                             v
HSI Pilot Side               HSI Copilot Side

 

2 devices + 1 shared gauge: master device mandatory

GPS #1 Pilot Side            GPS #2 Pilot Side (Master Device)
      |-----------------------------|
      v                            
HSI Pilot Side               HSI Copilot Side

 

2 devices + 2 different gauges + 1 autopilot: master device mandatory for the autopilot

GPS #1 Pilot Side            GPS #2 Copilot Side (Master Device)
      |-------------|---------------|
      v             v               v
HSI Pilot Side   Autopilot   HSI Copilot Side

 

Edited March 23, 20215 yr by RXP

3 hours ago, Nmcjr said:

So, you switch momentarily to get set up in advance then switch back, you aren’t changing the primary instrument. That’s one example where modeling the proper GPSS behavior makes sense , but mostly because practice makes permanent not perfect and I think those buying RXP are doing it for increased realism and if using for training I think it should work as it does in RW. 

Are you taking about a G500 here? Because in this case, I believe it is a factory setting to setup the PFD CDI source independent of the GPS CDI mode, but I'm not sure. Besides, are you sure the XP11 STEC 55X simulation is working like in RW regarding GPSS?

In all cases, the use case you're trying to get is very specific and I've honestly never heard of using the equipment in this fashion either, for the simple "safety" reasons @PeterBremer is explaining. I'm no authority about this, but I find this contrary to safety recommendations because the primary problem is loosing the navigational references, because they are diverging between what the aircraft is flying and what the pilot is seeing. This is a classic problem which has been solved long time ago in requiring, for a reason, to set the instruments in a way they are matching automation.

Nevertheless, I don't mind continuing this discussion and getting your input, and evaluate whether it is even possible to implement in the simulator. There is no guarantee though because of the SDK and the internal implementation of the autopilot system.

OK, I'm now clear of an NDA, so I can speak a little more freely here.  Its a for a new TorqueSim SR22 Entegra model I've been helping with testing on. So- I think there will be some additional sales for you on these units with this sim model as I think RW owners would be inclined to use the RXP navigators- but the behavior may lead to some confusion. 
I have confirmed this behavior on the stock XP430, and reference is made here: https://developer.x-plane.com/article/track-to-intercept-making-sense-of-lnav-and-locapp-modes-in-11-10/
"Thus, it allows you to arm localizer interception, while tracking a GPS flightplan that for example takes you through an RNAV transition onto the ILS approach. In this case, GPSS is active, the HSI selector can be turned to a VOR/LOC receiver, and then NAV or APPR mode can be armed."

Its a STEC55X and here are some key points: 
-GPSS gets its input directly from the GPS, not the CDI, so CDI mode is irrelevant in GPSS mode- right or wrong, its how it works, and is commonly used to get things configured in advance.
-The AP (in NAV mode only) sees the CDI information the same whether GPS or VLOC- it doesn't know the difference.
-Only GPSS mode can fly procedure turns and missed holds, NAV cannot- so it should not send those commands to the CDI. 
-GPSS cannot capture the glide slope/path, must be in NAV/APR mode
-so, you can see how there is some button-ology that needs to be practiced, and many folks have a hard time keeping it all straight, leading to some unpredictable behavior.

I realize some of this is limited programmatically, I'm just listing how it works for you to think about if its possible to model.  All in all I really like the capability of the RXP units.