CPU and GPU rendering | Rendered viewport Preformace

Hello, so i got two things that i would like to point out.
First, is there any way to use the GPU and CPU to render at the same time? i know i can open two blend files, set one to GPU, one to CPU, render them, and then combind the images later, witch will give a better resoult than if i would spendt the same time rendering with only one of them.

is there an addon for this? and if not, why wont they add this feature to blender?

And second, is there a way to get better preformance while using rendered view in the viewport? in most cases, i would like to prioritise preformance over rendering speed while working with the rendered viewport.

You should be able to set a scene to use the cpu and another scene to use the gpu, then combine each in the compositor as separate render layer input nodes