I know there are airports with more than one ATC Tower, though I only know it from my own country (SCHIPHOL - EHAM). This airport has two towers called Tower-Center and Tower-West. Tower-West is built because of a sixth runway (18R - 36L), that wasn't clearly visible from Tower-Center. (They found this out after completion of the sixth runway.)

How common is it an airport decides to built a second tower, and are there a certain rules or limitations before the decision can be made?

Next to that; what are the practical consequences for pilots and ATC's? Do they not only switch between startup- and ground-controllers but also between several ground controllers?