I've been doing some orchestral transcription work in my spare time, and one of the things I've noticed is that while double stops are relatively common in violin, viola, and cello parts (along with triple and occasional quadruple stops), they are rare in the double bass parts. In fact, I don't think I've ever seen one.
Why is this? Is there something about the playing of the double bass that makes it especially hard? Or is it just one of those things that "just isn't done"?
First I’d like to point out that all the info provided by @BenCrowell is on the money. I would just like to add a few points from the perspective of a bassist.
The bass does lend itself to some limited double stops and the best by far are 5ths. They give a nice powerful growly sound. The instrument is tuned in 4ths and since you can only bow adjacent strings at the same time and the notes are very far apart the only practical double stops excluding open strings on bass are from a m3 to a P5. The problem with double stops on the bass is most that fall in the practical range of the bass are encroaching on the “low interval limit”, which is a basic guide that determines where specific intervals start to sound muddy or indistinguishable. Divisi is also sometimes used but usually in octaves, 5ths or playing independent lines.
A bass virtuoso can play many kinds of double stops, either using open strings or in the upper register but in an orchestral setting it is best to let the bass “hold down the fort” and leave the harmonizing to the upper strings, who are much more adept at it.
In writing for strings, the double stops you typically see are thirds and sixths written for a violin solo. In a violin concerto, for example, this gives the soloist a chance to show off some harmonic color and sound fancy. On the violin, these intervals fall comfortably under the fingers of the left hand, and a proficient player can easily play them in tune, and with vibrato if appropriate.
If you have a big chord with a lot of notes, there is typically no reason to have a string section play double-stops. They're going to sound better playing divisi. The strings are supposed to be the main, sustaining part of the orchestra that is almost always playing. Double-stops are a special effect, and it's not idiomatic to have the strings playing them a lot of the time.
Some intervals are awkward to play as double stops. Fifths are awkward to play on violin, viola, and cello, because the two fingers can't be at the same position, so you have to use a single finger across both strings. For similar reasons, fourths are awkward on double bass.
In symphonic writing, the kind of standard default would be that the basses and cellos double each other an octave apart. I don't think a double bass can play an octave double stop, except maybe in the extreme high positions.
Most chord voicings use wide spacings for the lower notes. So if you look over the possibilities, probably the most reasonable double stop to give a bass player would be a fifth or a sixth. I'm not a bass player, but I believe the sixth would normally have to be played in thumb position. But there's no obvious, common reason why you wouldn't just give the top note of these intervals to the cellos.
(along with triple and occasional quadruple stops)
String players roll these rather than playing all three or four notes simultaneously. This is more the kind of thing you'd write for a soloist, and basses don't often get solos. You hear triple and quadruple stops, for example, in the Bach cello suites or in the Telemann viola concertos.
