A simultaneous Front Snap kick and Heel kick moving in reverse to each other (resembling the cutting movement of scissors) is called a ‘Scissors’ kick, it is executed against the side of an opponent’s knee and ankle.
Just like a bicycle, one leg moves up while the other goes down. In this case, there are two different actions available. The lower foot hooks the ankle/foot and pulls up while the upper foot pushes on the knee to cause him to fall backward. The second reverses the process with the lower foot pushing against his ankle while the upper foot hooks and pulls on his knee, causing him to fall forward into your arms. Both are best executed just before his advancing foot touches the ground.