As a sound guy too, I can tell you it's a simple matter of physics. Mid frequency and high frequency tones "blend" better. Chords in sub frequencies (with rare exception) sound horrible.
So, if you had two bass players, they would have to occupy two different sonic footprints. Either...
One would have to play low bass and one higher (which really negates the need for that higher instrument being a bass).
Or one could play sustained notes while the other does plickity plackity stuff...which can usually be handled by a drummer/percussionist.
If you'll notice, the same has happened throughout history of music. Babershop quartets don't have two bass singers, while all of them have a couple or three guys in the middle/high range. Symphonies usually have a slue of mid/high instruments with maybe a couple tubas or double basses. Those bass tones simply clash when layered in most cases. (There are exceptions, of course, but they are rare.)
Short version: They couldn't realistically occupy the same space like keys or guitars can. If they have to occupy two different sonic spaces, why have them both be basses when so many other instruments do the mid/high things so much better than basses do?
Just an FYI, barbershop quartet consists of Bass, Baritone, 2nd Tenor (melody), 1st Tenor (high harmony). Just one of each is the norm.
