Taekwondo and Judo are so different that in a Taekwondo class, throws never enter the dual practitioner’s mind, and, in Judo class, he never even accidentally kicks or punches the opponent. In a street attack, the dual practitioner would instinctively use Taekwondo techniques against a striking attack, but would revert to Judo if grabbed. However, if a person is a practicing black belt in, say, Taekwondo and Aikido, there is a problem. Taekwondo stresses linear, powerful, hand and foot strikes, and hard blocks, whereas Aikido stresses circular deflection and avoidance techniques, and uses soft blocks, locks, and few strikes. When he trains in the individual arts, the dual practitioner may be able to keep the two arts separate, but during a surprise street attack, which art will he use instinctively?
After training to be forceful in one class, and then training to be yielding in another class, which art would the practitioner use instinctively? He would probably use some ineffective combination of the two. In addition, how does a person be an expert in, and fundamentally believe in, differing approaches to the same thing. If one believes linear, hard techniques are best, how may the person also profess to believe that circular, soft techniques are best? It’s similar to a person saying he believes in and practices both Christianity and Buddhism.
Since there is so much to learn in their professions, doctors, lawyers, engineers, etc. specialize in one field of their profession. Whereas, martial artists seem to be able to become masters in numerous fields (styles) of their profession. Either they are highly intelligent, super athletes, or it must be relatively simple to get master certifications. With the advent of the Internet, master certifications are easier to get than a driver’s license.