Intriguing.  Looks very nice.

Couple of questions:

can the boards still stack easily with this back to back cooler arrangement?

can someone with some watercooling experience give me an idea of the BOM to cool 12 boards?
12x80 = 960W

It seems like a great idea but I'm worried it will prove expensive when you add in radiators, pumps, fans, reservoirs.  Is this going to need £300 investment on top of the cooling blocks?