NVEnc is dedicated hardware, it has little to no impact on the GPU's 3D performance so there is little to no reason to use separate GPUs for gaming and streaming. Using a dedicated PC for streaming means your main GPU has to output something your capture card can handle, which in turn means you need to cap your FPS no higher than that. You don't have this limitation when streaming from the same PC you are playing from.
If your gaming rig has enough spare performance to handle streaming, you would likely get better overall game and stream performance doing everything on a single system.