[Bluecobra]

If you are buying servers at scale the costs will certainly add up vs. buying two processors. If you buy single proc servers, that is double the amount of chassis, rail kits, power supplies, power cables, drives, iLO/iDRAC licenses, etc.

[dboreham]

You can build motherboards with two or more completely isolated sets of CPU and memory, that are physically compatible with standard racks etc.

[Bluecobra]

Good point, I forgot about those. It would be interesting to see if 1x PowerEdge C6525 with four single processor nodes is cheaper than 2x Dell R7525 servers. The C6525 does support dual processor, so it does seem a bit wasteful to me.