Toshiba is reportedly developing a tablet with two 7-inch displays, which folds like a book. That would make it the latest in a long line of companies-both big names and start-ups-who have toyed with these dual-display devices.

The concept makes some sense. A design with two displays maximizes the screen real estate while keeping the size down. You can mix-and-match different display technologies and operating systems to serve different applications in a single device. And the success of touchscreen smartphones, and more recently the Apple iPad, demonstrates that many users are willing to forgo a physical keyboard. But so far the dual-display has been DOA.

Asus, HTC, MSI and Sony among others have all experimented with these devices, either as full-blown tablets or e-book readers, but there's no sign any of them are coming to your Best Buy anytime soon. The Microsoft Courier project generated a lot of excitement, but never made it out of the incubator. One Laptop Per Child scrapped its XO-2 dual-screen tablet, and instead plans to release updates to its standard XO netbook. OLPC will eventually offer a standard low-cost tablet, the XO-3.

Then there are the dual-display e-readers. Start-up Kno demonstrated its device at the D8 conference, but its unwieldy tablet, which consists of two 14-inch displays, has puzzled reviewers. The Entourage Edge is a hybrid device-the company calls it a "dualbook"--with a 9.7-inch E-Ink display on one side and a 10.1-inch LCD tablet running Android on the other. It's a novel concept, but the device is much thicker than an e-book reader or tablet, and doesn't really excel at either.

There is, however, one very successful device with two displays: the Nintendo DS. (Yesterday the company announced the Nintendo 3DS, as anticipated.) Why has the DS worked so well? It has great software designed specifically for the two screens, including a touchscreen, as well as the stylus and buttons. The result is a dual-display device that is easy to use, even for young kids. (It also helps that the DS is meant to do one thing really well.)

This sounds easy, but it isn't. Recently I set-up a Nook, the Barnes & Noble e-reader, for a family member. Unlike Amazon's Kindle, the Nook has a second display, a small color LCD touchscreen, designed for a few basic tasks. It's a nice feature, but everyone who tried it found that the interaction between the two different displays was confusing at first.

Aside from Apple, no company has yet figured out how to deliver a great experience on a standard tablet, let alone one with two displays-sometime using different technologies and input mechanisms. Google is still working on versions of Android and Chrome OS tailored for tablets. And Microsoft CEO Steve Ballmer recently admitted that the company has a lot of work to do on Windows 7 tablets to catch up with the iPad.

Developers may create apps for these platforms, but they are unlikely to spend a lot of time and money customizing them for a dual-display device unless the potential audience is really big, such as with the Nintendo DS. Without really good software tailored to them, dual-display devices will be no match for e-readers, tablets or netbooks.