I am not with Laminar Research; just a flight simmer.
X-Plane is resource hungry with reliance on the processor and the graphics card for best performance.
If you add a second graphics card you will diminish the "power" of the processor in which case you need a "top end" Intel processor. Anything AMD provides a diminished output.
You also need to consider your motherboard. Does it have sufficient number of connections that will allow you to connect and run the additional graphics card?
What is the capacity of your power supply. I am of the opinion that you may need to have a good power supply to run your planned total system. Other reference sites also indicate the need for a good power supply.
My new flight sim setup with Air Manager will be a new dedicated PC just for X-Plane and my previous X-Plane PC will be just for Air Manager. There are several references available on YouTube whereby the knowledgeable PC builders run two PCs for the same scenario you propose.
Good luck with your setup