A powerful effect with both magician and audience.
Two spectators each select a card, and two different numbers. The chosen numbers match the location of the card they chose. Next the deck will be shuffled by both members of the audience.
Lastly, the total of the two audience's chosen numbers matches the location of the first card.
Best of all, the spectators can shuffle the deck, and count by themselves. The magician never knows the numbers chosen until the trick is finished.