I know this about something from a while ago, but i just recently caught the fight from the first round. In the Henderson-Pettis fight, (i think it was the first round) Henderson does this two hit kick. One of the commentators said it was a taekwando kick. it was like a leg kick, followed by a head kick while still in the air. any idea what its called/how to do it?
It's commonly called a double kick.
The key is to turn the hip less and retract the leg on the first kick as you jump and twist your hip for the second.
Isn't really practical to apply it to Muay Thai type kicks, so a fighter would have to already have an understanding of the basic TKD round kick.