I found this site http://www.rijwiel.net/kruisfre.htm a while back while looking for some history on Mixtes myself… It’s based out of the Netherlands, and from what it explains in a kind of round about way, the mixte is a leftover from a group of bike frames referred to as Cross frames, or Girder frames, or Truss frames.

The main thing I found interesting on the page, is the image of a page of bike frame drawings in the middle of the page. It is supposed date, from what I have gathered, from a periodical (Magazine) article written in 1901... Some of those frames probably never made it to production, but if you look at frame number 33, it sure looks like a classic Mixte to me… So that would make that frame style known at least in conception clear back to 1901...