You may think that the Samsung Continuum was another dual-screen phone, but it actually had one big screen that was split into two parts. Verizon launched this phone in 2010 as part of the Galaxy S family. The navigation buttons were in a strip painted over the lower portion of the screen, leaving a small sliver at the bottom. This "second screen" could show notifications, news, and other information. As interesting as the basic concept was, the implementation was lacking and didn't make up for the wasted space.
Interestingly, the same basic idea is behind the Galaxy Note Edge, though not the new Galaxy S6 Edge.