Ayreon is also an interesting band featuring multiple vocalists per track. The only thing is that you won't find the same two vocalists on different albums, since the guest vocalists keep rotating.
mastodon System Of A Down is probably the biggest i know of
The Blackout do. They're also one of my favourite bands. Here's a few of their more famous tracks;
Kansas has two singers.
The Libertines! Best band of early 00s.