There are 2 separate things. NVENC and OBS-Studio rendering on GPU.
For 2 or more NVidia graphics cards, NVENC can easily be run on the other GPU in video encoder settings. (0=first, 1=second)
OBS-Studio GPU rendering... isn't shown in OBS-Studio settings, it was there on old OBS-Classic. Have to quit OBS-Studio, change primary monitor to other GPU, start OBS-Studio, then switch back primary monitor. This might speed up and improve performance of video captures, HDMI capture card, and the game itself if GPU limited. This may slow down and harm the performance of game capture and limit display capture.
Ideally it is best to have a separate computer to stream with PCI-E capture card or USB 3.0 capture device.