Look, it is not the good idea to use different GPUs for game and capturing software or even running the game in SLI/Crossfire, because if obs uses one video card (to render the final output that will be encoded) - it needs to download images from other video card. This is possible when you using system memory as intermediate storage. The option to do this trick known by two names (v21.1.0):
SLI/Crossfire Capture Mode (Slow) for Game Capture source and
Multi-adapter Compatibility for Window Capture source (and Display Capture in case of win7). It downloads image from one card into system memory, uploads it to other card as texture, this texture combined to other textures, then final image downloaded into the memory again to be encoded by CPU, and then it can be send over network or stored on the disk.
More info:
"game capture" techniques from developer.