The Sridei Eish has a famous teshuva where he was asked about men and women singing shabbos zemiros together. He recognized the problem but was ultimately matir for the purpose of kiruv. He writes that the gedolim in ashkenaz where far more advanced in maintaining an orthodox religion in a secular society than those of poland who were far less tolerant. He tells of a visit by R. Yisroel Salanter to Germany where R. Ezriel Hildisheimer offered Torah classes to young women. R. Yisroel commented "if this would be done by a rav where he comes from he would be defrocked, and he should be, but he wishes that his chelek in gan eden would be with the tzaddik R. Ezriel". [Palo Alto 2008 = Germany 1850]
I am not sure if this story took place before or after R. Yisroel's own son became a mathematics professor and stopped practicing religion.
So, in that teshuva (1:87 - in the older printings) he writes that when he originally came to berlin he was surprised to see this practice of singing zemiros together in chareidi homes, but he mentions the famous sevara of תרי קולי לא משתמעי and somehow applies this to the context of kol isha. He says that he was originally not so excited about the heter, but then found that the sdei chemed also quotes a sefardi rabbi who is matir singing zemiros kodesh together.
The gemara on today's daf, especially with Rashi's comments (d.h. k'eish) seems to clearly indicate that even if there would be many women singing together, it would be forbidden for men to listen to their singing. True, it is not speaking of zemiros kodesh, but why would that make a difference?