Eric, thanks for the picture.
There are a couple of explanations, the first is a common neutral bus bar (shouldn't be case here, Six2 knows better); another is neutral grounded inside the box (ditto); the final one is leakage in the Edison to PowerCon cables. I've seen shop-built cables trip GFCIs due to less than ideal assembly practices....
You can test for the first 2 by using your ohm meter to check for open or short between the neutral slots of the red and black circuits. The neutral should be the taller slot, typically in the 9 o'clock position. You should find this to be an open. A short indicates that somehow the neutrals from both inlets are tied together. Test for ground bonding by checking for short/open between the neutral and ground pin for both Black and Red circuits. These, too, should test as open circuits.
That takes us back to the cords you're powering the distro with.
Tim, I had an employee once.... once. He never assembled cabling though... more of a self proclaimed "A1 mix god" after 25 shows.
The distro feeder cables are made by me. Photo attached. No stray strands or obvious (to me) issues. No continuity between terminals.
I recently went through my pre built CBI powered speakers cables too. Some need some light fixing like re stripping to get the connector strain relief back on the cable housing, but they were all functional and there were no blatant mis-connections or "only 1 strand still connected" situations.
The distro has separate neutral bars for each circuit. The red circuit's neutral bar has only 2 taps, 1 coming from the input and one going to the first red receptacle. Then the neutral daisy chains from red receptacle #1 to red receptacle #2. On this distro, the 2 front red receptacles are the only receptacles fed by input 2.
There is NO continuity between circuit 1 and circuit 2 neutrals (white wire/silver terminal). As expected, there is continuity between the ground/green terminals as the ground bus bars are connected at the chasis ground screw located on the left, behind the circuit breakers. See pic for test setup.
-Eric