Let me throw-in my calculation.
In image processing algorithms such as ones used in cameras we do have linear scaling. So adding second CPU gives almost 100% more processing bandwidth. In most cases workload distribution is extremely simple - schedule processing to first free CPU. In short: two Digics - twice FPS.
Since 1DX has 14FPS
(with mirror lockup, see here
) and 18MP it means that one Digic5+ can do 126MP/s
If 5Dmk3 would have 22MP with 7.5FPS it would need 165MP/s.
It's either two Digic5+ (not probable), significantly less per-pixel processing (even less probable, since processing is done in ASIC it would new Digic HW version) or either MP or FPS is incorrect.
Just for comparison: using 18MP sensor with one Digic5+ gets 7FPS which is optimal from development point of view (reusing sensor from 1DX and one Digic5+).