Nice build. I assume both coolers' fans are blowing toward the back of the box. Does the rear cpu run a little warmer than the front because of this?
I have a couple of Noctua NH-D9DX i4 3U coolers running horizontally, with fans mounted on outside of both radiators. Pulls air into rads from center and blows up and down, so no warm air from either radiator is pulled and/or pushed into another radiator.
When installing I just thought they might work a little bit better that way, even though they look a bit strange:
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