Finally, after many tries and much learning, two clearly working
JACK servers, cooperating, on one motherboard:
https://github.com/jebofponderworthy/MultiJACK/blob/master/README.md
Stéphane Letz did point out quite a while ago that the design goal
(getting more JACK DSP power out of one motherboard) probably would
be served by use and addition of the loopback driver to a
single-JACK configuration. I didn't understand it then, but do now,
and will be testing this first on the next major iteration of my
synth platform:
https://lsn.ponderworthy.com