The two optical-GPIB units (digital controllers in vendor parlance) communicate independently of the power supplies, not just a pass-through.  They continue communicating, even sending status information when the supplies are off/disconnected.  The remote/local status relates to the units, not the supplies.  The ioc <-> comm unit control loop may be completely separate from the comm unit <-> power supply, so we may or may not be able to talk to the supplies directly, hopefully it would be the same protocol, but we don't know that.

The first five DIP switch settings on each unit sets the address.  Switch 8 means upstream link (first in chain).

In troubleshooting I reset both units a number of times, power-cycled both supplies and experimented with the DIP switch settings.  I assume resetting both comm units recovered things.  If the problem re-occurs we should see the comm status on the screen flash between OK and NO COMM.