extreme imbalance in the exhaust pipe output

I am still working on my custom EFI installation, but in the process of noticing my rear bank cat was red hot, I also noticed a substantial imbalance in the perceived flow of exhaust out of the tailpipes on the two sides of the engine.

I took out the cats, and using a blower, discovered that the single muffler, although it has 2 inputs, and 4 outputs, the two sides are separated, no mixing of the two sides at all, two distinct mufflers in one large package.

The output from the front bank of cylinders feels much stronger than the exhaust output from the rear bank of cylinders, and this stronger flow is much hotter, very strange.

I am still wrestling with the imbalance between the two cats. I am going to check the backpressure in both sides next.

any thoughts ? This one is a real head-scratcher. Almost seems like the front bank of cylinders is running full blast, and the rear bank is just going along for the ride. The rear bank does get hot, and yesterday the cat lit up like a light bulb, but I reduced the richness of the mixture and that cat did not get hot, but the other one did....

I took out the plugs and the injectors, even flow tested the injectors they seem to be all the same, no front-back bias there in the injector flow >............

any thoughts ?