I found this source "Virgil Thomson: Music Chronicles 1940-1954" which refers to the Milhaud pieces as a "double-barreled work" or "double piece."
And every other mention of this technique I can find also refers to the Milhaud composition, so maybe it's simply not common enough to merit its own jargon!
It is technically polyphony and counterpoint as well, but that doesn't address the real idiosyncrasy of the subject, that it's two separate pieces that work both in isolation and when combined.
This seems like a blue ocean opportunity for inventing your own name for the concept, I do like "double-barreled" but how about something like "combinatorial composition" or "modular music?" (We already have the "mashup" genre for unintended combinations, but I assume that's outside the scope here.)
EDIT: Apparently "modular music" is already a term of art and it seems applicable to Milhaud's work: https://en.wikipedia.org/wiki/Modular_music
Modular music is music that originates from the combination and overlapping of different compositions one over the other. The compositions —also called modules— are written by one or many different composers in different moments. New modules can be added or removed to create a totally new work, a new composition, different from the original one.
It's even cited as the "technique" behind the invention of counterpoint in the Medieval era