If you want to move two cards (or more) as one -- while having your spectators see and believe there is only one card, then your actions must the exact same as when you are simply moving one card. This takes practice, practice and more practice.
While you talking, and looking at your spectators, square the deck with your right hand, as you do this, lift the inner ends of the two top cards with the ball of the right thumb, pull them a little to the right (as in Figure. A) -- and leave them in that position.
Next, as you direct the audience's focus to the top card, move the tip of your left thumb against the middle side of the two cards and push them out, as one card, over the side of the deck. Hold them between the tips of the index finger and thumb, turn them over and place them just above the deck but not on it.