System of a Down - I really love Daron and Serj's voices. I like Serj's a little more because it's for heavier stuff, and it's the side I love from System of a Down, but their harmonizing is amazing, I think. Linkin Park - Well...I think they're the best rap/rock group ever because of the fact that Mike has an amazing rapping voice and Chester's voice for rock. It's the perfect combination. Alkaline Trio - I love the way they switch vocalists on Crimson and Maybe I'll Catch Fire. ..................AND POD's few songs on Testify with Matisyahu are great with the way they switch...Not really a band with two vocalists but I thought I'd mention I like it.
I know that they've been mentioned about 3895268 times in this thread already, but I have to say that I love Taking Back Sunday. Also, I agree with Branden. The Postal Service and The White Stripes are both awesome. I love the way Emmylou Harris sounds when she sings with Bright Eyes, even though she isn't technically part of the band. Damien Rice is really good with that woman he always sings with. Rancid and Flogging Molly are my two favorite bands ever, and they both have multiple vocalists. Classic rock bands have the best multi-vocal sounds. The Who, The Band, Fleetwood Mac, The Grateful Dead, and countless others. That post turned out way longer than I had expected it to be.