re: Bands that had two great female vocalists/songwriters

Ann and Nancy Wilson - although we don't hear Nancy enough (I mean, the band had Ann, after all) - she was more than competent as a lead vocalist, when pressed, and a great backup singer for her big sister.

On the other hand, Nancy also had outside contributions to film scores and other music giving her musical credentials indepedent of her sister and the band Heart (or their side project, The Lovemongers).

I think Heart qualifies, particularly if you don't qualify "vocalist" with "lead vocalist" (again, although Nancy did sing some lead, most notably on the hit "These Dreams" - coincidentally turned down by Stevie Nicks.)