I'll wager the second gen can will be taller to accommodate a second Xeon CPU - The New Mac Pro Duo!
A taller, two-CPU version.
A second Xeon CPU would require a different system design. two CPUs and two GPUs won't cut it power , shared thermal core , or single fan wise.
Xeon E5 1600 v3 line up will probably go 6 , 6 , 8 , 8 cores. The E5 26000 v3 line up may top out at 15 . The current 12c variant is a 3 stack "layer cake" design.
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http://www.anandtech.com/show/7285/intel-xeon-e5-2600-v2-12-core-ivy-bridge-ep ]
Taking the three stacks of 4 to three stacks of 5 shouldn't be a huge problem for a more optimized design for the 22nm process. At worst they could drop one a count of one on one stack to 4 to lower the L3 cache pressure. (similar to how the 1680 v2 disables two cores but keeps that layer of L3 contribution. )
What two CPU packages used to do is
a. enable a high number of cores. Well 14-15 cores is alot for a very large group of folks. The folks that are left in the 16 & up range for individual usage workstation isn't that large.
More economical access to 6-8 core is a far bigger issue and potential winner.
E5 v4 (broadwell) could easily be looking at 20 cores top end in a single CPU socket. ( 4 stacks of 5 which is quite similar to the current 2 stacks of 5 hooked by QPI to each other only all inside one package. The CPUs are like two black holes in close proximity. One gets absorbed into the other at some point. )
b. RAM capacity. There are two ways to get more capacity. More DIMMs or denser RAM chips. Moore's Law says the latter is on track to getting better over time. In 2015-2016 the density is going to be up. Which means the need for dual just to get to triple digit GB capacity is going to be down and sinking.
c. [ with move to E5 generations ] could add more PCI-e bandwidth. The Mac Pro isn't on that path. Apple left the "six slots" club well over a decade ago. It is extremely doubtful they are coming back with some new Mac Pro design. Trading SATA lanes for more PCI-e bandwidth can easily uncork the current Mac Pro's lane oversubscription problems without going to two CPU packages.
The core theme is that next generation implies the future and the future is unwinding the constraints that two CPU packages tended to solve.