As Otto says, there are a number of folks with more than one sub. Some people use two subs to achieve both high SPL's and low frequency response (sweeping generalization: the deeper a sub is tuned, the less overall sound power level it typically can provide), especially in rooms wiht very large volumes. There are also cases where the most convenient places to put a sub yield room nulls (areas where the way the sound waves reflect off the walls and other obstructions in the space yield "dead spaces"), and adding a second sub in a different location can help correct those nulls.