What kind of screw is it this with two different intertwining threads?
That type of threading is called hi-lo, or sometimes high-low. 

From MutualScrew.com:


  High-low screws -- or hi lo screws, as they are often called -- have double-lead threads (where two grooves have been cut around the shaft). This kind of screw includes a high thread and a low thread. The unique qualities of hi-lo thread specifications help minimize the amount of material displaced as the screw is driven into the material, while also serving to create a stronger grip than would otherwise be possible.