From: Rabbi Moshe Sofer
To: Rabbi Shraga Feivel
In a community where women must travel three hours through dangerous mountain roads to reach a mikvah, returning at night poses serious danger. The Chasam Sofer endorses a double hefsek taharah strategy — performing two hefsek taharos on days four and five of the sighting — so that the seventh clean day coincides with the eighth day from the first hefsek, allowing daytime immersion on Friday. He rules that one who is strict will be blessed and one who is lenient has not erred, leaving final practical arrangements to the local rabbi's discretion.
Source: Shu"t Chasam Sofer YD §197