Does it make sense to have a dedicated low-end GPU only for the monitor to save rendering-power on the second GPU in the system?
My thought is to implement a low end Nvidia quadro - so i also would have the benefit ot 10bit color display.
But if my understanding is right then the viewport-performance would suffer under such a configuration or is there an option to choose which GPU i want to have for viewport-performance?
In my experience it is a good thing to have dedicated GPU for display. The computer is much more responsive during rendering and you can use more of the GPU memory for rendering. I would not however implement a quadro card because in blender there would be no benefit over a geforce and so you would be wasting your money (unless you use software that runs better on a quadro card). 10 bit color display is also not a real benefit if you are doing 3d graphics - AFAIK blender does not support 10bit output and you need to know how your image will look on standard 8bit monitors. If you have troubles with banding it is better to solve it with dithering.
The only problem is to found out which GPU to buy to have the best vieport performance. There are unfortunately no reliable benchmarks testing the blenders viewport performance…