What do you mean by passthrough here? Usually passthrough refers to passing through a GPU to a virtual machine. And there is no cooperation whatsoever required between the GPUs for that. That makes me think you’re talking about offloading: one GPU controlling the display, while the other does the heavy lifting of 3d rendering. Last time I checked - several years ago - that is impossible with the proprietary nvidia driver, unless you have hardware that supports that, like prime in laptops. The only way to do offloading to a nvidia card without such hardware was to use the open source driver nouveau. And at the time there was absolutely no point in offloading with nouveau because it had such terrible performance. Now, this might have changed on several fronts since then; so take it with a grain of salt.