The first Duplex Typewriter model (sometimes referred to as the Dennis-Duplex), was invented by Adolphus. S. Dennis and patented in 1890. It was made by the Dennis Duplex Typewriter Company of Des Moines, Iowa starting in 1893. This typewriter had two keyboards, each with its own set of lowercase characters while the uppercase and punctuation marks were divided between the two, so that the typist could potentially type twice as fast by typing two characters at once. If this sounds over-produced and unnecessarily complicated, it was. The typewriter did not do well and its unique keyboard layout was scrapped.
Have one for sale? Please email me at Antikey.Chop@gmail.com