LG Voyager in late 2007 as a featurephone replacement to the OG Apple iPhone, which was then an exclusive for AT&T. The device had a 2.81 inch resistive touchscreen on the outside, while flipping open the clamshell body revealed a non-touch 2.81 inch screen and a physical QWERTY keyboard inside. And of course, there was the Kyocera Echo which was launched in 2011. With two 3.5 inch screens available, combining them made up a 4.7 inch display which seemed pretty large in 2011, but not today. Part of the problem was with the device's thickness, as you can imagine, and finding developers to write apps that could take advantage of a second screen. Unlike HTC's patented side-slider, the second screen on the Echo clicked into place when the device was opened.
HTC seems to be on the right path with the HTC One which has a fabulous design. The last thing the company needs to be worried about now is selling the public on a new dual-screen concoction.
source: USPTO via Engadget