Running two computers requires having two batteries, motherboards, etc which means more weight or means less battery life for the weight, whichever way you think of it the user is losing because of this choice. The idea of having two computers in one is one of these cool technical ideas which just won’t provide a benefit for the users.
The best thing for the users would be to have a single light-weight computer that provides an adequate amount of performance. A 1GHz ARM processor should give good performance when running Linux for most tasks (web browsing, office applications, and most games). So it seems that a good tablet with some USB ports and a matching USB keyboard would be a better option. Maybe you could have a spare battery integrated with the keyboard as most times when you want a full sized keyboard weight isn’t such a big problem.
The problem with MS-Windows in regard to such machines is that performance is poor and processors other than i386 and AMD64 have never really been supported (sure you can buy MS-Windows-based PDAs that have ARM and PPC CPUs, but the application support for them is minimal).
These problems will never be solved. MS wants to compel users to continually upgrade their software, this requires always adding new features (bloat) while an older or less featureful OS is often a better option for lesser hardware. Vendors of proprietary applications will never support the range of CPU architectures that a free software distribution such as Debian or NetBSD supports (not that NetBSD is an ideal tablet OS – but it does demonstrate what can be done if you want a portable OS). These inherent flaws in the proprietary software environment lead to some unusual hardware being designed to work around them – such as the Lenovo U1.
I don’t have any reason to believe that the Lenovo is a bad machine, it sounds like a reasonable work-around for some of the problems that MS-Windows has forced on the industry. But I believe that it would be a much better machine if it was lighter, had a better battery life, and had a choice of keyboards – all of which would have been achieved if it was designed as an ARM-only tablet machine that you can connect to an external keyboard. They could even have used a multi-core ARM CPU. But the market for such systems apparently isn’t large enough for Lenovo (or anyone else) to ship ARM laptops for serious use. There have been a couple of netbooks released recently with CPUs that don’t support the i386 or AMD64 instruction set, but they were aimed at web browsing use not general purpose computing. Almost everything that I do on computers could be done at least as well with a CPU that doesn’t run an Intel-based instruction set, the only exception is virtualisation which doesn’t seem to be well supported on architectures other than AMD64 nowadays.