Thanks John for the clarification and feedback...and thanks to nyteowl, I have taken your comments and modified my drawing to make more sense...thanks for your help with that.
The GCX manual, on PAGE 8, outlines switching 2 preamps
to one power amp, but those preamps will most likely share a common ground so no hum will ensue...There is no specific diagram to outline switching 2 Guitars
to one amp so that's what my original Diagram set-out to explain. Since 2 guitars will not share a common ground, and the RTN jack is ground-lifted, if you do not use the ins as outs for switching 2 guitars, as in my drawing, you will have hum from one of the guitars.
I have to admit that it is hard to get your head around using ins as outs and vice versa, so, I added a configuration to switch 2 guitars to one amp using the IN and RTN jacks as inputs and the OUTPUT jack as the output for clarity.
But, as you mentioned John, since the RTN jack is ground lifted, there may be a requirement to connect the Shield connections of the OUT and RTN jacks to eliminate hum/ noise from the 2nd guitar.. (my new above diagram notes this).
So, if you are using the configuration where you are using the IN and RTN jacks for the Guitar inputs, you will require a wire or a break-out box to make the ground connection between the RTN and OUT Jacks as shown below...)
Thanks for your help, I think that all of the wiring options are covered now!