What makes a good double lift, and how do I perform it?
That's a good question which has an annoying answer. The reason it's annoying is because the answer is different for each person.
Everyone develops their own style of technique, so each move performed should blend into your existing styleof card magic. If you tend to handle playing cards in a way that looks like you don't have any card skills (See the card magic of Lennart Green as an example of this), it will look very odd to suddenly perform a complex flourish as you turn a playing card over. Each move you perform should be invisible. Making every action look natural, and in keeping with all your previous actions with the playing cards, helps achieve this overall impression.
What to do:
Pick up a deck of cards and turn a single playing card over. Don't think about how you will turn the card over, just do it.
A good double lift should look EXACTLY like that.
Anything you add to the action, weakens your technique. Film yourself performing the simple action of turning over a playing card. You want to record your natural method, before you start playing with that action. Refer back to it as you learn the double lift. The closer you can get the sleight to resemble your natural action, the better your double lift will become.
How to learn the double lift:
If you are looking for expert tuition on a double lift, including a range of magic tricks that use the move as their core technique, these sources may be ideal for you:
It's important that you choose a source of instruction that gives you some card magic tricks that use the double lift. Once you drill the technique in isolation, you need to be able to rehearse the move within the context of a full magic trick or routine, This will help you to develop the timing and flow of the move.