Natalie Cole and Diana Krall just recently recorded a cover version of "Better Than Anything". If you have good jazz chops, then this would be a good song for two very good female singers to do.
There are thousands, but my personal favorites are from Wicked (Tracks: Popular, Defying Gravity, For Good, Loathing, etc) and Take me or leave me from Rent.
I Know Him So Well from the musical Chess, made famous by Elaine Paige and Barbara Dickson. The Boy is Mine by Brandy and Monica. Sisters are doing it for Themselves by Eurythmics with Aretha Franklin. Any songs by Shakespears Sister. Any songs by Alisha's Attic.
There are many female singers in the world. I can't mention without knowing. The female singers whom i know are Smt. Lata mangeshkar, Smt. Asha Bhosle are good singers for Hindi films. These are very old singers. And other singers are Smt. Suseela, Smt. Janaki old singers for
Malayalam films. Smt. Chithra is the latest singer.