Josiah,
You can control in more than one ARTCC, but you must be assigned to only one. Basically, you can join Los Angeles, for example, but also control in New York or even more than two ARTCCs as a visiting controller. The only caveat is that you must spend atleast 50% of your time controlling in your home, or assigned, ARTCC.
You can read the actual policy on the VATUSA website.