It is better to floss twice a day instead of once a day?

Once is sufficient. For most individuals, once a day is fine. If one has food traps between the teeth, then extra flossing to remove the food is good. Once a day thorough cleaning to remove the biofilm will help protect against decay, gum & bone disease and breath problems. I like to see individuals also using closys mouth rinse to improve their effectiveness in controlling the biofilm.
Once or twice. Flossing disrupts bacterial colonies so they cannot become organized to start to cause gingival or tooth problems. As long as you floss properly and not aggressively sawing it around your teeth and gums, twice a day is not harmful.