Check out Eyes Set To Kill. There is only like 2 girls in it,but one of the girls does a lot of the singing. They are pretty awesome. Young Blood Spills is a pretty good song. And they are like really hard metal. The guy does a lot of screaming for the band and the girl sings. It sounds really good together :) -You can also look on the site http://funadvice.com/r/3js7cinel2 (no .com). You can search by a band name and find other bands like it. For example,if you typed in Flyleaf,you could find other bands that are like them. Best chance of finding metal girl bands :)
Epica is nice Nightwish (old singer not new) Evanescence (not really metal but meh) Kittie Incubus Sucubus (Old songs were metal new ones not so much) Otep
... I'll think of more...
Madder Mortem are good... There's a whole list I could write but I'm sure you would know all of them lol.
not really heavy metal but try florence and the machien probably best current female british singer
idk about led metal but evanesence and Flyleaf are some personal favorites... Flyleaf especially!
ahh the site link didn't post right...
www (dot) last (dot) fm
Delain! How could I forget Delain!? Bloody awesome band!
Arch Enemy
flyleaf