It will probably set the secondary card using the auto-select process. It should say right underneath the drop-down. You can manually select the second card, but I don't think you'll have to.
It will continue to leave your 470 as the primary either way. With the 430 selected as the PhysX card, you offload PhysX processing to the 430 so the 470 can perform even better than if you just used the 470 alone with a PhysX-enabled game.