Origin of dance/techno remixes by more than one person being listed as "vs."?

I’ve noticed that on a lot of songs in the dance genre, other remixes on the LP that are made (or at least I assume this is what it’s referring to) in collaboration by more than one person, the two names are listed as “X vs. X”, e.g. for the song “More than This” by Madison Park, the whole album is listed on iTunes as “Madison Park vs. Lenny B”. Or one of the remixes for the song “Million Miles from Home” by Akira, listed as “DJ Manian vs. Tune Up!”. This seems like a very strange way to express that two artists are collaborating on a piece. Why isn’t, for example, a slash used instead?

Valete,
Vox Imperatoris

Not quite an answer, but back in the early 80s when dub reggae was big there were quite a few albums released by people like Scientist who either had a side each of tracks, or even alternated tracks.
iirc Scientist vs Prince Jammy had 5 tracks a side, alternating between the artists, so they both got the first track on one side.
The cover of one lp I recall (might even be the one above!) had the artists dressed as fighters in a boxing ring and the tracks were listed as Round One, Round Two, etc. and introduced by an announcer saying Round (X) and ringing a bell…

These days, do they maybe mix different parts of the same track, instead of each having their own?

I always assumed it was a play on the “MC contest” thing. You know, these producers are so up for it they’re literally brawling for the right to twiddle a few knobs.

I think you are on the right track. The oldest “vs.” references I can find are all from old dub albums. The oldest (so far) is Fatman Dub Contest - Prince Jammy vs. Crucial Bunny from 1979.
If we look at the issue from another angle, it seems that most “vs.”-titles are from remixes or mashups, and AFAIK the dub tradition is one of the oldest popular “remix”-forms out there.

Scratch that. The oldes dub album that uses the term is Rodigan v Williams - Rockers Arena from 1976.

Well, I think I actually have the Prince Jammy vs. Crucial Bunny lp somewhere, but I’m certain I don’t have the Rodigan vs. Williams one… Don’t think I recall the sleeve, either - if it had been about a few years later I would probably have picked it up but 1976 was a bit early for me buying reggae.

The practice of releasing two-artist albums with titles including “vs.” actually dates as early as 1964. Vee Jay Records released an LP called “The Beatles vs. the Four Seasons” that year, consisting of one side of Fab Four tunes and one side of Four Seasons tracks. Vee Jay only had the American rights to the first Beatle album, and repackaged it every way they could.