Boxing is about punching: boxers have a reputation for amazingly strong punches, well known among all fighting sports. The reality is simple: boxing is limited to 4 basic punches to be delivered with large padded gloves and that has helped the techniques and the training to evolve to its maximum efficiency.
On the other hand Tae Kwon Do is a martial arts (and fighting sport) that is nearly fully based on kicks. TKD fighting allows full contact for kicks and, quite strangely, controlled contact for punches to the head.
For a long time I kind of assumed that if someone could do boxing training and then TKD do training would be a good puncher and then a good kicker, therefore a good kick boxer. A few weeks ago I had the opportunity to realise that my thinking was flawed when LK joined my club. He was fitting perfectly to the above description: 2 years of TKD and over a year of boxing.
While in principle this person knows how to punch and kick very well I immediately noticed a peculiar behaviour in his approach to sparring: he would either kick, nice combinations, while maintaining a pretty poor guard, typical of TKD fighters; in other circumstances he would get close and punch like a real boxer. It then occurred to me that his training, his knowledge of fighting, was lacking a fundamental part of what kick boxers rely on: articulated combinations of punches and kicks so he would do either or but never combined.
Ultimately, because sparring should be spontaneous and without thinking his subconscious either relies on the punching combinations he known from boxing or from the kicking combinations from TKD but he never mixes and bridges between the two. It will be a while before alternating kicks and punches will be natural to him.