Linux

You have some good answers and some bad answers here.

It’s not the fault of the people answering, what you’re asking has been piecemeal and scattershot in implementation over the last decade so everyone has some bizarre response they came up with to be happy.

Allow me to share mine: use a kvm switch.

The switch lets you plug two computers into one keyboard, video, and mouse. But you’re gonna just use the video part. Plug it into both your motherboards and gpus video ports and push the button to switch back and forth between the gpu for gaming and the motherboard for everything else.

Why only gaming? Because everything else you reference can make use of a gpu that’s not being used for video. I guess some game engines support rendering frames and then sending them to another output device but that’s not something to rely on.

So when you’re using blender you see the model on your monitor plugged into the motherboard but the heavy lifting is done by the gpu. When you transcode a video the same thing happens.

I came to this solution after trying to do what you’re asking for in x11 and having a bunch of headaches about it everytime an update would come down.

Pushing a little button on the desktop was easier than messing around with software to make a rube Goldberg contraption to do the same thing. Mine had two leds on either side to indicate which “computer” I was using at the time. I ended up wrapping electrical tape around the rim to cover them both up and cut out the word “turbo” from the tape over the green led that indicated I was looking at the gpu.

@jeena As always the Arch wiki has you covered. PRIME, is the system you need. It's slightly easier with the monitor plugged into the iGPU, (that's how laptops are wired).

I'm going to assume that it is possible to put both the dedicated and integrated GPUs to work, though I've never seen this kind of setup.

This is likely not something you want to actually do.

The integrated GPU in your processor is not an additional bit of computing power your computer is not using, but special software that can use your processor to put out graphics if a dedicated GPU is missing. It is extremely inferior at processing graphics compared to the real dedicated GPU, and if you were running firefox to watch (Not decode) youtube, you would very likely see things like screen tearing as the processor struggled to keep up.

If you wanted to do this just to see the outcome for yourself, you could switch your displayport cable to your monitor to connect to your motherboard instead of your GPU to get an idea of how rough this would be. If you wanted to continue after seeing this, I believe you would need to connect a 2nd monitor in order to use both the dedicated and integrated GPUs.