I've said it before here and I'll say it again: I'm always impressed when someone says they practice both Chang Hun Taekwon-Do and Kukki Taekwondo. Why? Because both styles have rather different body mechanics (if we're talking about official ITF and official KKW TKD, anyway). I'd personally say they're different martial arts with the same name, although others might say they're just two different styles of the same system (or two different systems of the same style, however you want to view these fairly arbitrary appelations). Anyway, it's an impressive thing to become proficient in two distinct styles. In that sense, learning the forms for each style is good because they're part of the system.
That all being said, I've never actually met anyone who does practice both types of TKD. I've met many people who practice both ITF and KKW forms using the same body mechnics. In effect, they've simply doubled the number of forms they know, but they perform all the forms in the same manner so they haven't learned two different arts or styles. They've just made more work for themselves, that's all.
Forms teach one how to generate power, perform specific techniques, demonstrate strategies, condition footwork, impart the style in a systematic way so the student has an easier time digesting what they're taught and letting one form build on another, etc. They're invaluable when it comes to traditional MA training, IMO. To teach two (or more) pattern sets without focusing on the different body mechnics is simply to miss part of what is being taught and in many instances engages in needless repetition because of the overlap in techniques between pattern sets that occur since there's no difference in how they're being performed (in reality, for example, there is quite a bit of difference between an ITF side piercing kick and a KKW side kick but I've never seen any of the people I know who "do both" make any sort of differentiation between them when performing the different forms).
Pax,
Chris
