Math trick for 2 spectators

This trick needs 52 cards, without joker, which the spectator can shuffle. It works best with 2 spectators.

Start by asking spectator 1 to deal cards on the table and to stop whenever they want. You need to know how many cards were dealt. If that's an odd number, spectator 1 can remember the last dealt card. If that's an even number, they need to remember the card on top of the deck, which you can then deal on the table too. They can finally put the rest of the deck on top.

Now, calculate your key-number : take the number of cards dealt and divide it by 2 (remove the decimal part if needed). Spectator 2 can then do the same thing, but you now don't have to re-calculate a key number.

Both spectators can now cut the deck, but not shuffle it. Next, ask one of them to deal 2 piles on the table.

Take the first pile and ask spectator 1 if their card is in it. If yes, move cards from the top to the bottom of the deck corresponding to your key-number. If no, do the same thing with the second pile, but add 1 to your key-number.

Finally, you can ask both spectators to take their respecting pile, to deal the top card next to it, and to stop when they see their card. The 2 spectators' selected cards will appear at the same time.