The reason for representing Shaka Nyorai and Taho Nyorai in pairs is based on the following anecdote in Kenhotohon, the 11th chapter of Hoke-kyo Sutra.
釈迦如来と多宝如来を一対で表わすのは、法華経の第11章にあたる見宝塔品（けんほうとうほん）の次の説話に基づく。