just getting a chance to follow up.....
this is how I wired it:
per each tortoise - all in the foreground to terminal strips, all in the background to terminal strips. I used 4-conducter cables to connect the two terminal strips as shown using a coupling visible in the dangling cables.
it only worked from 1 side as described at the beginning of the thread. I thought I stumbled on the issue as described above - thinking the switch was on the wrong side of that double dashed line....
I have it working now, but the ONLY difference is the terminal strips are connected via the continuous 4-conducter cable WITHOUT the coupling. Absolutely no difference schematically.
but...it works, so I will leave well enough alone. so now if/when I need to renove this section it's 5 minutes with a screw driver instead of simply disconnecting at the coupling