If a husband attaches divorce to the menstruation of both wives simultaneously ('If you both menstruate, you are both divorced'), and one claims menstruation while the husband denies the other.
Chapter on Explicit Divorce and Others
Al-Mughni
Book of Divorce
Primary text
If the husband affirms both claims, both are divorced. If he denies both, neither is divorced, as each divorce is contingent upon two conditions: her own menstruation and the menstruation of her co-wife, and one wife's claim is not accepted against the other. If he affirms one and denies the other, only the denied wife is divorced, as his affirmation validates the conditions for her, and the unverified claim of the co-wife does not establish the condition for the affirmed wife.
Supporting text
If the co-wife claims menstruation, her statement is not accepted, as her knowledge of another's menstruation is treated like the husband's knowledge of it. If the husband denies the first wife's claim, she is divorced based on his admission of her own claim.