How to get display output on secondary GPU (non primary)

Hi all (:

tl;dr: i need help to get display output from my 2nd gpu (which is connected via x370 chipset.)

So it looks like the bios of the ASUS X370 Crosshair HERO is bugged and the primary BUS switch does not work.
No matter which settings i choose the 1st GPU is always the primary display adapter.

Curiously the LUKS passwordpromt is shown on every GPU.

But after that Linux (Manjaro/Arch) is does only use the primary gpu (maybe others don’t even get initialized?)

Is there any way to force linux to use another GPU as primary display output?

with kind regards

Ive never got it to work but after i switched to a APU/iGPU setup i saw this post (:
Maybe it helps someone in the future <3