[Anthony]

Realistically, two skills that default to one another probably have a shared core of common fundamentals, and then there are specific details that you only learn under the one skill. To give a sports example, epee and foil fencing have a great deal in common, but someone who's only learned epee won't have learned the right of way rules in foil. The question for defaulting, then, is what portion of skill improvement comes from improving shared fundamentals, and what portion comes from learning tricks that are specific to the narrower field; the first will improve all your defaults, the second won't.

The only method GURPS has of representing this sort of thing is Techniques; there is no generic group skill bonus (which would represent training the fundamentals). However, simply capping defaults doesn't really help, many skills really are mostly about fundamentals.