A follow-up to this thread:
I did finally manage to find an online copy of the WX-500 installation manual, Revision E.
In it, it is mentioned that the WX-500 may be able to be connected to two displays via RS-232, tying the RS-232 RX lines in both displays to the TX lines in the WX-500 processor, but leaving one of the displays disconnected from the RX lines on the WX-500 (in other words, the WX-500 can send to both displays but only one display can talk back to the WX-500).
For people who have done this with the G3X Touch system, do you know if this works? Is it necessary, i.e., will the GDUs share WX-500 data between each other without connecting both directly to the WX-500?
If you have done this, do you have the MFD or the PFD GDU set up to talk to the WX-500? Does the GDU send magnetic heading or ground track to the WX-500?
The manual also states that connecting directly to the aircraft's heading source is preferred but I'm not sure that's possible with a G3X--the G3X installation manual, Rev AP shows only an RS-232 connection, which I assume provides either heading or ground track to the WX-500 processor.