I don't know that dual woofer subs are inherently better than single woofer ones. I've heard incredible (and expensive...) single woofer subs that absolutely astonished me. I've also heard dual woofer models that sounded like crap.

I suppose it depends on the individual components involved. Out of curiosity, are you running some sort of positional setup with your dual subs or do you simply have a Y-splitter sharing the signal?