Thanks everyone for the replies above!

Informed by those ideas, and in hopes of giving myself some further clarity on the issue, I'll try to summarize below what I presently see as the main use cases for multiple sequencers (note what follows are not strictly exclusive of one another):

  1. more sequencing lanes & CV outputs: simple idea here, yes, but sometimes you just need more
  2. running different instruments or instrument groups: the most standard instance of this would be percussion on one sequencer, pitched on another
  3. complementary sequencer styles: for example, a "normal" sequencer vs. an acid specialty sequencer vs. a "euclidean" sequencer like Rene 2, etc. A good list of sequencers with style labels is at: https://doudoroff.com/sequencers/
  4. derived sequencing: requires 2 or more sequence signals run through logic (e.g. AND modules, etc.) to output a derived signal. This hits on what @Lugia was indicating in his 1st post above. Tons of ways to do this, potential modules to involve, potential patching. In general, this gives a good way of getting some complexity and change in an output signal from two pretty basic input signals.
  5. additive sequencing: not wholly distinct from other techniques mentioned here, but worth spelling out, this is adding gate signals to construct more general stepped CV signals, see https://www.modulargrid.net/e/forum/posts/index/9928 for a very interesting detailed example
  6. function-specific sequencing: an example of this would be like @Ronin1973 indicated, e.g. where one sequence chain is driving pitch CV and another is driving gate/trigger signals. The main premise here is, for a single instrument voice, there are multiple sequence inputs driving the score and realization
  7. compound sequencing: sequencers feeding sequencers, as if SeqA outputs go to SeqB inputs, or SeqA outputs go to SeqA inputs (if it was capable of that kind of feedback). This is kind of a bottomless pit topic, and surely depends on the specs of the sequencers involved

Of course one or more of these use cases could be combined at any given time, if the modules support doing so.

[This post is edited to shorten... additional ideas / comments later.]