So, if one has a lot of things happening, multiple JACK
instances could make more use of available resources?

I don't think that follows.  That seems to me that you are asking if since
you have a lot of processing going on, would adding additional overhead
help.
What I want to do, is to use the resources I have to run multiple signal generation and processing chains asynchronously, in parallel, and then use the final audio-hardware-synchronized chain to resample them all into one, perhaps using the Zita tools.  Anyone know if this is possible?  I saw this flow structure work very well in the video domain, quite a few years ago.

If you have multiple processing chains that do not depend on one another,
then using jack2 may help, it can take advantage of multiple processor
cores.
JACK2 is indeed in use.  Things are nowhere near as good under JACK1 in this setup.

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