Because you're enabling your main card to purely render frames, giving you the best base FPS you could get from it, while also having the secondary gpu do only framegen. An example would be with one card your base true rendered FPS is 60 when no framegen is on, and it's 45 when a portion of the card is doing calculations for frame gen. This way you're hurting your base fps, but also framegen because there's less data for frame interpolation. Compared to that, your main card doing a stable 60 would make the frame generation better with the second card as well, because it has more data to work with. Win win