Dual vocals?

Old Carcass. The vocals on Necroticism & symphonies of sickness are ace. Older Theatre of Tragedy is very effective blending female vocals and low vocals at the same time.(same with the first within temptation). Older Orphanage has growling/singing male vocals with some female vocals.

Tristiania has male singing,growling with female vocals. I think the band has 3 seperate vocalists. Listen to the song 'the wretched'.

All I can think of right now.
 
Ram-Zet!

Haha and I think Nile has 3 vocalists. I can't hear the difference at all.

Then uh, The Flower Kings has two lead vocalists, one of whom is Daniel Gildenlöw. Nightingale uses dual lead vocals on most of the songs on their album Invisible, as well.
 
Im pretty sure carcass did some on Necrotism, and i believe some of their latter works like Heartwork and Swansong eg/ Songs like Black Star