When performed correctly, the double-lift is one of the most powerful and useful techniques in the magician's repertoire.
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.
Place your left fingers so that the cards are placed between the ball of the thumb on one side, the first joints of the second & third fingers on the other and so that the tip of your index finger rests against the outer edge. The two cards are now squared and can be displayed to your audience. (Figure. B).
Now take the cards by the lower outer corner, between the tips of your index finger and thumb. Your thumb's tip covering the lower index, and turn them face down on the deck. Place the cards so they line-up perfectly with the top end of the deck, but do not release the hold of the thumb and finger. With a small up push of the thumb tip on the face of the lower card, release it and move this top card away to the right.
Understand that the above technique takes only a split second. The card is turned and immediately swept off the deck smoothly, the lower card being left squarely on the top.
Generally, when this card manipulation is used in a magic trick, the changed card is immediately planted into the deck. The actions described above moves the card over the side of the deck, with its back to the spectators -- so placement into the deck follows very smoothly.
Tips: Practice, Practice, Practice... then practice some more. Do this in front of a mirror many times until you can even fool yourself.