Hi

I think it might potentially take advantage of 2 CPUs but basically vMix mainly uses GPU. I never could have 100% CPU load with an i7, so I'm not sure it's really interesting except if you want to run multiple external encoders/recorders on the same machine.

That is because a i7 processor is at it's max above 50% and will start to drop frames around 60% because windows calculates percentage over the threads instead of the logical cores.
Hyperthreading can get around 10% extra margin, but I like to keep my system well under 50% cpu usage.
For lots of tasks, like the multicorder and recording in other formats than H.264 with hardware encoding, vMix is very depending on CPU and memory bandwidth.

I am quite happy with the Newtek codecs though, as they take much less cpu when multicording a lot of inputs.