@phaedrux
That was it. Apparently the RRF Config Tool doesn't attach F0 to the tool by default and is supposed to be assumed, but the explicit definition is required for it to work properly with two tools.
@seventhwhiskey one other option would be to provide an external 5V supply and turn on the 24v via the PS_ON pin and a relay.
That way the config will be run before there is 24V to spin the fan.
Other benefit is the Duet can shut down the PSU if it detects any heater faults
