A little (unplanned) research turned up an interesting fact. When the Novus Ordo Missae came out in 1969, the date of November 30, 1969 was presented as the first day it might (not must) be used, and “Priests who celebrate in Latin, in private or also in public, in cases provided for by the legislation, may use either the Roman Missal or the new rite until November 28, 1971” (Paul VI General Audience Nov. 26, 1969). (I recommend reading the general audience document because it reveals some concerns about the use of the vernacular.)
The two rites were to exist side by side for about two years. At this time they were referred to as the “new” and the “old” rites. In some places, a person might find the older rite, but the newer rite was to become more and more common and eventually replace the old.
Pope Benedict distinguised them as “forms” of the one Roman Rite rather than as separate rites. Thus, they now exist side by side again. This is not new! It was meant to be temporary, but now the situation is back, in some ways, as it was from 1969-1971. There is a key difference, however. The “new” is the “old,” and the “new” is not meant to overtake the “old” but to cause inevitable changes, which cannot exactly be foreseen at this point.
I bring this up because I had not thought about the situation in the past where the two rites (now distinguished as forms) were both in force at the same time–two “options,” if you will.