It defaults to "auto-select", which means it will use the card with the least load, which will 99% of the time be #2. You could just select the option for card #2, but why? I just leave mine at auto with SLI enabled, and best performance as the preference. See no reason to change unless I pick up a lesser powered third card for PhysX, but I don't see the point now, or in the future at this point.