[gnicholas]

The Half Keyboard from Matias does just this. You type with your left hand, and when you want to type a key from the right side of a keyboard you use the space bar as a modifier key. So to type an 'L' you hold space and hit the 'S'.

In addition to the Half Keyboard, which is literally half the size of a regular keyboard, they also make full size keyboards that can be used in this way. I bought a Half Keyboard years ago, for around $100. They now sell for $600, likely because they're making money on ADA-motivated purchases. Employers are much less price-sensitive in those cases.