|MadSci Network: Earth Sciences|
This is a question students ask a lot. The short answer is no, two hurricanes could not combine. The way the atmosphere behaves, and the physical laws that behavior follows won't allow this. This question and answer are posted on USATODAY's Hurricane FAQ page, along with some information about what happens when two hurricanes begin to interract. The most interesting thing that happens is called the Fujiwhara effect, where two storms circle each other. You can read about this at USATODAY pages also.
Try the links in the MadSci Library for more information on Earth Sciences.