It's more than character-specific knowledge, it's mastering the game through one character, to the point where you're competent enough with the huge amount of fundamentals that you can effectively transition to a secondary. Just because you happen to know all of the Link tech you can know, can do it on cue, and can do it with razor-accuracy in terms of input, does not mean you're exhibiting a mastery of the game. Character knowledge and tech knowledge will only get you so far. It will take years to develop your gamesense. It's a lot more than playing a character for a week, knowing their tricks, and deciding you're experienced enough to move on.
The way you apply those mindgames, the way you utilize your fundamentals, are going to vary hugely from character-to-character, and you will never (read as: 'never ever') truly have a developed gamesense and mastery of said fundamentals if you're constantly spreading yourself thin between two characters, such that you're never utilizing either one to their fullest.
I've got all of the time in the world to improve my game, and no improvement or serious skill developments happen 'fast from the bottom', which is why I have completely dropped Link as a secondary, and am now going to focus on learning the game through Sheik, and not just learning how to play my character. Trust me, focusing on one character helps.