There shouldn't be any real issues with paralleling the power supplies, especially if they're the same series. It really isn't that much different that paralleling phases in a polyphase supply. The biggest problem you will likely run into is one power supply will be getting them to share the current equally. Paralleling the rails of a multi-rail design likewise isn't a big deal, I've done it myself feeding two different rails into the same GPU.
Getting custom cables made is more of a PITA to parallel things than you probably think it is though. It really is just easier to toss two PSUs in there.

Similarly, there's a huge difference in price and lead time with choosing the stock radiator dimensions that all the closed loop cooler vendors are using vs getting your own thinner and deeper one made. I really don't see an issue with their design, though I don't know why they don't just mount the intake for the PSUs on the inside and then exhaust through the back in typical fashion. Room temperature air has a typical heat capacity of ~34J/(ft^3*K) and a little manipulation gives ~1.75 °C*(ft^3/min)/W. Even if there were dumping 1200W into the case, it would only take ~200cfm to keep the air inside the case 10°C warmer than the outside air. That's well within the operating parameters of any half decent PSU.