Shuffle and cut the deck, squaring it up when you are done. Squaring up a deck refers to when you make sure all the cards in the deck are in a perfectly neat pile.
Riffle through the deck until you reach the top. As you riffle, your thumb should be motoring almost one card at a time, so it should be somewhat easier to feel when you reach the top two cards.
Keep those top two cards subtly separated in the back of the deck.
NOTE - If you cannot feel the two cards as you riffle through the deck, you will need to use a pinky break to do so.
A pinky break occurs when your pinky props up the cards in the back, one at a time, until you have two elevated. The end result will be the same as when you riffle through the deck, only the pinky break can be easier for some people.
Using your thumb and index finger, turn the two cards over, presenting them as one card.
Reinforce that it is only "one" card by asking the spectator to memorize this card (singular).
Turn both cards back over being sure that they always look like one card.