I just have to drop in again to say that following Justin's advice on another thread re the Duck Change, I have now all but mastered it, and I am so damned pleased with myself! If anyone else keeps having the issue of the cards spinning out sideways, take Justin's advice of placing your second finger a little way in from the top corner of the cards. So many of the tutorials I've seen suggest that you should have it right on the corner, barely holding the cards securely. This is what causes them to spin out to the left (assuming you're holding them in your right hand). You need to have a very light grip, but it does need to be secure. Justin's tip of practicing with one card is also gold. Just work on getting that one card to do the "flip" first, before you even think about adding a second.
If I could sum up what cracked it for me it would be that second finger position, and being as light as you can with the pressure applied to the card(s). Don't over-bend them. Just enough to get them to pop out when you simultaneously release from thumb and fingertip with gentle pressure from the forefinger. Also, don't hold your right hand too far from your left. Use trial and error to get it as close as you can while still leaving room for the turnover. This helps stop the cards separating on landing. But seriously: practice with one card at first. You don't even need to try to catch it in the other hand, just do it against a desk top, close-up mat, couch you're sitting on... keep trying to gently ping that thing downwards until you get it. Like most subtle sleights this is one of those where you have to manage it successfully (or nearly) once in order to get that "ah-ha!" moment. Then you know roughly how it should feel and you can reproduce it more easily.