According to the Frodsham chart, Walsh only made 104 duo-in-unos over the span of 34 years for Frodsham.When they were in use specialists made them. A. P. Walsh and Hammersley probably made the majority between them. In Mercer's book on Frodshams he lists the chronometers with 3rd party maker's names made for Parkinson & Frodsham. https://awco.org/present/SamHammond.pdf
In some instances Walsh only provided the hairspring, in others he finished the chronometer and in a few he also did the adjustment for trial. I am pretty sure that Hammersley's business must have operated the same way.
If the market for these were small enough, it would have been difficult for others to compete with two top level artisans although many at that period of the last half of the 19th century were probably quite capable.
This must mean these were tremendously expensive back then. So if I'm to understand correctly, the duo-in-uno was essentially for those who wanted a thinner pocket chronometer.