Here's the chapter on chord keybords http://billbuxton.com/input06.ChordKeyboards.pdf from Bill Buxton's work-in-progress book Human Input to Computer Systems: Theories, Techniques and Technology http://billbuxton.com/inputManuscript.html . Steno keyboards https://en.wikipedia.org/wiki/Stenotype are chording keyboards. The Velotype http://www.velotype.com/en/ and Veyboard http://www.veyboard.nl/en_main.html use a similar system that would be a lot more suitable for general-purpose text input. Then there's the important niche of one-handed chording keyboards, like the one Doug Engelbart designed for the NLS system, and the mobile, wireless Twiddler3 https://twiddler.tekgear.com/ .