Why don't 2 GPUs double the graphics performance of a computer compared to a single GPU?
When you are using two processors, not every task your GPU's encounter is parallel by nature. There are a certain portion of tasks which are strictly serial, and can be processed by only one processor at a given time.

If a set of tasks were to be 100% parallel your dual GPU set up should give double the speed (in theory).

A little mathematics for you: Ahmdal's law for speedup is given by 

$$S = \frac{1}{[B+\frac{(1-B)}{n}]}$$, where B is the percentage of tasks that is serial in nature, n is the number of processors and S is the speedup.

Put B=0 and you wil get S, ie Speedup=2.

Practically dual GPU gives us maximum 80% speedup.