iacisrmma,
The Yeshivas Chachmei Lublin asked the Gerrer Rebbe who reffered the shaila to Rav YM Tukaczinsky. And the Mir asked their Rosh Yeshiva, Reb Leizer Yudel who asked the Chazon Ish. They came back with differing answers as is well known, but most only fasted one day because of the pikuach nefesh aspect. A few yechidei segula fasted both days.