Seeing you like Fairport Convention, Steeleye Span is similar, one of their lesser known albums Hark the Village Wait has two female vocalists:
Steeleye span - lowlands of Holland - YouTube
And only really like one of her albums Sarah McLachlan's Fumbling Towards ecstasy
Sarah Mclachlan - 01 Possession - YouTube
And 2nd that Hank Dogs 'Bareback' album, two vocalists in the Fairport style: