It’s an important aspect of your magic to ensure that the spectator understands that when you turn over a card, it
is a single card in their mind; despite how it may multiple cards as one. With that being mentioned, you will most likely not want to use over-the-top, fancy double turnovers; but merely one that resembles your natural singular-card handling. Granted, it can be difficult to find techniques that match, but then it may be a good idea to perhaps change your handling of your single cards to match that of your doubles.
However, in usual circumstances it does not really matter what technique you use – but rather down to how you use them. To be honest, you could quite easily get by with a mediocre double turnover, as long as your posture and attitude displays itself correctly. For instance, if you were to perform the move casually without a moments thought, it would fly by fairly well. However, if you perform the move with much apparent concentration, with flashy and flamboyant techniques, it can begin to make your spectator think “
how much effort should it really take to merely turn over my card?” This eventually leads them to believe that ‘something’s happened’ even if they are not entirely sure what. [This is something that you do not want in your performances, as it can quickly turn into a battle between the magician and the spectator; with the latter looking for moves, and the former covering them up. This contrasts greatly with how it should be; with both parties enjoying each others company and having fun together.
The bottom line is, as long as your onlooking spectator cannot identify that "something's going on" with a mere flipping over a card, then you will be absolutely fine with whatever you choose to do.
Warm regards,
Jordan