Both volumes are contained in this PDF:
The following files can also be downloaded, but this is unnecessary if you’ve already downloaded #6460.
The priest who translated both volumes into English was Fr. Francis Brunner, C.Ss.R. Years ago, he was a “major player” in the Sacred music movement. He wasn’t a huge fan of the Solesmes rhythmic signs, probably due to his German associations. When I posted Brunner’s gentle condemnation of the hymn “On This Day, Oh Beautiful Mother” it caused quite a uproar …
BEWARE! There’s a version of Jungmann’s Missarum Sollemnia being sold which combines Volume I and II … but all the footnotes are missing!