"Ryoto tetsuritsu" refers to the situation where the family line of a country's monarchy is split into two and the monarch is alternately selected from these two family lines.
両統迭立（りょうとうてつりつ）とは、一国の君主の家系が2つに分裂し、それぞれの家系から交互に国王を即位させている状態をいう。