I pulled out my dad's old briefcase, and rediscovered his grandparent's joint passport - issued to my great-grandfather and including his wife - they are pictured together. Issue date 1960. Stamped with half-dozen or so visas from various european countries. When was this valid, and when did it stop? Couple interesting notes: we can deduce she would not have been able to travel without him... She was a professor/phd, and possessed a notable strength of character and progressiveness. Only his occupation is listed.
On my own, I found this from archives.gov: "Although 95 percent of mid-19th century passport applicants were men, many women also traveled overseas. If the applicant was to be accompanied by his wife, children, servants, or other females under his protection, their names, ages, and relationship to the applicant were stated on the passport application. One passport was then issued to cover the whole group. Likewise, when children traveled abroad solely with their mother, their names and ages were indicated on the mother's passport application. Passport applications by women in their own names became more frequent in the latter part of the nineteenth century, and by 1923 women constituted over 40 percent of passport applicants."
What this doesn't say is if through policy or law this practice has been discontinued, and when and why.