Dual monitors, IGPU, GPU

Solution
According to your monitor, you should be able to:

GPU Multi-Monitor
Select disable to disable the integrated graphics when an external graphics card is
installed. Select enable to keep the integrated graphics enabled at all times.

So for primary use PCI Express and enabled iGPU.
According to your monitor, you should be able to:

GPU Multi-Monitor
Select disable to disable the integrated graphics when an external graphics card is
installed. Select enable to keep the integrated graphics enabled at all times.

So for primary use PCI Express and enabled iGPU.
 
Solution