The speed of the processors will not increase, so 2 x 2 Ghz does not equal 4 Ghz.
You will double the amount of computing resources (cores/threads), but is not an exact 100% increase in performance. Some CPU cycles are used to manage the 2 CPUs, so you don't get a full doubling in performance
Your processors will still only operate at the level they did before, you'll just be able to do more things at the same time with the additional processor.