The idea of combining two chromatic harmonicas into one, usually a C and G tuned instrument, must have first come up in the 1920s. My assumption is that the first company to actually manufacture such an instrument was F.A. Böhm. However, I was not able to confirm that by any patent. The first double chromatics had external springs. which were very common in the 1920s. F.A. Böhm also produced a double chromatic for Gretsch around the same time.
Later models like the Schlott double chromatic had internal springs.
There is also a similar model made for Gretsch. It is part of the Alan G. Bates Harmonica Collection at the National Music Museum of the University of South Dakota. Bates also wrote, “This double-sided chromatic harmonica in C and G was a short-lived experiment. Most chromatic players became adept enough to play in any key on a C-tuned instrument, so a second key was unnecessary.”
Recently, I saw a picture of another double chromatic by the Emil Friedel harmonica company. The fact that this model seems to be rather post-war, maybe around the 1950s, contradicts Alan Bates’ assumption.
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Chromatic Harmonica History 