For this specific case ( diameter and height )? It won't physically fit. The second CPU package is almost useless without another set of DIMM slots.
Go take a peak at the "memory" section of the Mac Pro overview. (hmm, managed to find a static link. So
Image
http://www.apple.com/mac-pro/
)
The DIMMs are about as tall as the whole logic board the one CPU is sitting on. Where going to find room for 4 more of those? That single logic board the CPU is sitting on can't hold another CPU. The stuff you can see that you are moving out of the way would more likely double than be able to shove aside somewhere else.
Even if you could "steal" the logic board space from the space one of the GPU cards is taking up I doubt QPI links are going to traverse logical board boundaries well. So would likely have to be same board.
For a dual CPU logic board you'd need a substantially bigger power supply (if still want dual GPUs also ) , bigger fan (wider and/or more ) , taller ( two different DIMM stacks ) design. Probably would need two different thermal cores as the GPUs wouldn't be as tall as the extended CPU board. The glut of PCIe lanes would beg the question of just one storage device. Overall, it is probably at the tipping point of flipping back to a rectangle and multiple fans to get things to fit well.
Three major power consumers gets you a naturally to the triangle shape. Since need to cool both inner and outer sides of the triangle it is easy to over both with a larger circle. Hence the core design depenency flow.
four elements start to push at more rectangular shapes and solutions. Two thermal cores with circular tops bound by an enclosing rectangle.( like a house chimmney for a furnance & fireplace ).
It is more so it would have required two different cases with different fans , power supplies , thermal core(s) , etc. Detached from the single CPU package model the number of duals sold probably were not viable to Apple. That isn't particularly "new" since even in old design Apple didn't float two different cases for the "Mac Pro" class solutions. The CPU+RAM daughterboard was designed to promote reuse between single/dual configs.
