Double planes booster multiplies all the planes on the game field by two. Two planes flying over the screen at the beginning of the level confirm that the booster has been applied successfully. If you match two planes, you'll launch five, each hitting its own target. You can also see that the booster is applied by a different color of the planes on the field.

Note that double planes don't show up on the field, like Bombs or Rockets. They only affect the way the planes you already have on the field or get by matches work.