Double Taisei: The Sun and the Stars (Double Taisei #1)

Genius shogi player, Taisei Tendo has a split personality. He is the well-behaved "Tai" during the day, but turns into the freewheeling "Sei" during the night. Tai is about to participate in the professional qualifying tournament for shogi, but his world is suddenly turned upside-down when he switches places with Sei!