First off, I'd suggest doing a song that both of you love deeply, and arrange your harmony for it so you're not doing a version of a song another female duo has made well known.

But for ideas on who has done it well & inspiration...
Sally Rogers & Claudia Schmidt Closing the Distance
Anne Hills & Cindy Mangsen Never Grow Old and Never Grow Up , though they do have some guests performing with them. Lost Jimmy Whelan is a lovely spooky one
Silly Sisters (June Tabor & Maddy Prior) - any album, but particularly Grey Funnel Line for acapella harmony
Janet Russell & Christine Kidd
Kossoy Sisters (check out sister harmony in general)
There's not near enough of it, but on some Trapezoid albums- can't think of which because they're tucked away on cassettes - Feyda Epstein and Lorraine Duisit did some piercingly lovely duets

Joanne in Cleveland