The µTron was designed for use with the thumbs, but you don't need to use them. It is ridiculously expensive, but not only is it more ergonomic than ordinary QWERTY keyboards and has mechanical Topre switches, it is also one of the most good-looking keyboards ever ... and insanely cool.
The Kinesis Freestyle is a rubber dome. Fully split into two distinct pieces.
The
Goldtouch is widely available. It is a high-quality rubber dome keyboard that has an adjustable ball that holds the two halves together. You can adjust the angles between the two halves and lock them in place. Laying flat, the two halves are separated by an inch. There is both an older and a newer version with slightly different layout for Home/End/PgUp/PgDown and comes in either (yellowish) white or black.
Although it is not split into two distinct pieces, it is certainly possible to screw it apart and remove the locking mechanism. That could give you a little bit more hand-separation. If you want more, then you would have to mod it with a new cable but unfortunately, each little wire is soldered separately (at least they were on my early version).
Even without any mods it is
much better than any Microsoft "Natural" keyboard. The keys can be a bit noisy though compared to other rubber dome keyboards.
Edit: The FreeStyle has a SRP at $99, and the GoldTouch at $129, but the GoldTouch is much cheaper on the second-hand market. Sorry about the confusion earlier.