Reason
Niou just loves the switches.
Being Yagyuu is fun, he says, under his buchou's mildly disapproving eye and Yanagi's barely-there twitch when they switch again on the courts. (Back? Or to each other, even the master does not know. This makes the both of them smirk, and Yagyuu-who-may-be-Niou curl his head a certain way that would hide his eyes if there were no glasses (but there are, that's almost the whole point) and Niou-who-may-be-Yagyuu laugh meaningly and swagger. They both shoot him little looks of smugness.)
Being Yagyuu is almost like being himself, only more polite and almost subtle. They are not opposites, they are mirrors, and Niou tells Yagyuu (one hot lazy afternoon with the both of them melting together, slick skin pressed together and laziness -reluctance of movement, the one with the glasses says- in the air) that after all, Yagyuu-kun, even though the other may raise a different hand, is it not also the raising of a hand? And Yagyuu had said, Really? Then, Niou-kun, are not complete indifference and changing the subject both ways of dodging my question? And Niou had smirked against the bare skin of the other boy's shoulder and rolled back on top just to hear the long-suffering sigh and how his partner can tell him it's too hot in a manner that's dignified.
Being everything that the other boy is, being restrained and deliberate and thinking in the long term about the consequences (the one who wears the glasses thinks about what might happen and picks the option that results in the least inconvenience, the one hanging all over him playing with his rat tail the option of least boredom) and really being Yagyuu is like bring himself only with different priorities.
In other words, not being himself at all. (Chop logic is another one of his specialties. Yagyuu assures him it is contagious.)
And what, says their bed-ridden buchou, is so fun about being each other?
Niou says, not really lying at all, watching your heads explode, and thinks, not really being truthful at all, not being me.
Niou says he does it because it's fun, but suspects he will keep on being Yagyuu past the time it stops being fun at all.
-end-