2 kicks | 10 times each leg | Repeat for 2 to 4 sets or more

Stand facing the bag at the distance of a kick
Step forward and twist your foot only one time then do the two kicks in a row.
Get back to your initial standing position and start again. Try to stand on your toes (the standing leg) and bring back the kicking leg to touch the ground quickly and lightly between the 2 kicks. Your standing leg should bounce but not twist between the 2 kicks.
This exercise is good to build speed, strength and balance for your double kicks.