I was just informed (in a catalogue) that in the Babylonian Talmud, Shabbat 61a, that Rabbi Ulla wrote "women and men are two separate nations". Who was Rabbi Ulla, and was there more discussion on this?
The phrase that you saw in a catalogue seems to have been quoted out of context and inaccurately. The reference that you write is from Shabbat 62a (and not 61a), and it was said by Rav Yosef in explanation of a different legal ruling that Ulla made with regard to a complex Talmudic discussion.
In the Talmud there (Shabbat 62a) it says that "women are considered a group unto themselves." This phrase is used in a technical discussion, dealing with the laws of carrying an object from a private domain to a public domain on the Sabbath (which is prohibited by the Torah).
The Talmud explains that clothing may be worn from one domain to another on the Sabbath because wearing clothing is not considered "carrying" an object. The question is what if a person wears something that is not normal attire. Is that considered wearing clothing and permitted, or is it considered carrying and prohibited. In this context, the Talmud says that it is prohibited for a man to go from a private domain to a public domain wearing a type of adornment that is normally worn only by women, because "women are a group unto themselves" and the normal dressing habits of women cannot be applied to men in order to permit a man to go from one domain to another wearing an item normally worn only by a women. In contrast, if one wears the type of clothes normally worn only by one sect of men, it is considered attire, and it is not called "carrying."
I hope this was helpful. All the best,
Y. SHAW