Vernon wasn't suggesting that you turn over your card like a flower. Why would anyone turn a flower over? I suppose you might want to use the stem as a toothpick, but that would be pretty unusual. The point he was making was that the double should be held delicately, with a minimum of force being exerted, and, as you say, with as little tension as possible. If you turn your singles over in an ugly, tense manner, then I think it would be an error to try and adapt your doubles to suit this handling. Also, different techniques for the double lift suit different occasions. Rather than making your double turnover match your single turnover, make your double and single turnovers match the best possible turnovers in that given situation.