The Wikipedia page on twelve-string guitars provides scant information for those curious about historical details of the instrument.
In particular, I find that the information regarding the twelve-string's origin strains credulity:
The exact origin of the modern twelve-string guitar is not certain; however the most likely ancestor is the Mexican bajo sexto, a six-course bass stringed instrument used in norteño music of that region, and traditionally tuned like a conventional guitar doubled at the lower octave (like a modern 8-string bass with two additional strings).Following the first link provides a slightly more realistic picture (emphasis added):
... [T]he 12-string guitar was developed by Italian luthiers laboring in the guitar workshops of companies like Oscar Schmidt, Harmony, and Regal in New York and Chicago. Italian music has a long history of wire-string, double course instruments like the mandolin and because many of the builders were of Italian descent, it would be a natural experiment to double the strings of a standard six-string guitar. One of the most famous 12-strings in the world has a strong Italian connection. According to family legend, Leadbelly custom-ordered his famous Stella 12-string from Fulvio Pardini, Who worked for the Oscar Schmidt company in New Jersey.The most likely story (to me) seems to be that the instrument was more inevitable than anything else. Classical instruments of ancient origin, such as the mandolin and the lute, involve strings paired in octaves or courses.
The other theory is that the 12-string arrived in the U. S. from Mexico. Latin America has a long history of double-course variants of the standard six-string guitar. These include instruments like the tiple, the charango, and the cuatro. Mexico has a particularly large number of guitar variations ranging from the diminutive guitarra de golpe to the massive guitarron.
Depending on how you look at it, twelve-string guitars are probably as old as guitars themselves. Lutes, which are a likely precursor to guitars, are traditionally strung in courses. Various guitar-like instruments have probably been invented and used across thousands of years, incorporating various numbers of strings strung in various courses or not. To describe the "origin" of this kind of instrument would be to claim a false starting point. Most likely, people have been experimenting with the infinite variations of strung-and-fretted instruments for 6000 years or more.
If you want to consider how far down the rabbit hole actually goes, spend some time searching YouTube for performances of harp guitars. Here's an interesting sample of what you'll find: