HI Mikey,
All good points from JohnK re sealed vs ported (I've never heard a meaningful difference about the quality of low bass from either design, although that controversy has gone on for years). You'll gain an extra 3 dB in output by using a second subwoofer--subjectively, 3 dB is "somewhat louder"--so at its low-frequency limit there should be more output using both subs. But it won't change the low-frequency extension, unless you got different subs and one was capable of deeper bass.
The main advantage of two subs is in averaging out the variations in low bass caused by the room resonances that I spoke of before, so you get smoother low bass with fewer hot spots and areas of cancellation. If you use one sub in a "difficult" room (like all of them!), and you sometimes just move a foot or two on the couch, or sit in a different chair, it will signifcantly alter the amount, smoothness, and quality of sub bass you'll hear. So if you set up two subs carefully, it's a favor to others watching the movie. Everyone will experience smoother and deeper bass from different locations in the room.
Go into the archives of SoundandVisionmag.com and check out Tom Nousaine's articles on subwoofers and their interaction with rooms. He's a friend of mine and one of the most knowledgable guys on subs.
Regards,
