I wrote about this a month or so ago. Basically, if you find the current state of DX12 satisfactory for your needs (some folks do, some do not) - just switch to DX12 mode and then pop out as many gauges and stuff as you like… Without the need for any type of secondary GPU/iGPU…without any FPS loss.
The primary cause of the FPS drop is that under DX11 we are limited by a single rendering thread being dedicated to its active rendered frustum.
Long story short - the moment we put even the smallest part of the sim on another display, it will automatically half the framerate.
There are ways to mitigate this under DX11 - such as the use of additional GPU’s like the integrated GPU as you write about here, or the additional system through the extension cards and various sorts of interconnect popular within the flightsim design and manufacture industry - yet as you can see it is all rather complicated.
DirectX 12 solves that in an elegant way, also being one of the first steps towards the integration of true multiple display support.