Observational study of double superoutbursts in long-orbital-period AM CVn stars. - Kojiguchi, Naoto; Isogai, Keisuke; Tampo, Yusuke; Kato, Taichi; Itoh, Hiroshi; Monard, Berto; Hambsch, Franz-Josef; Wakamatsu, Yasuyuki; Ohshima, Tomohito; Ito, Junpei (Publications of the Astronomical Society of Japan, 2026)