This article on the subject looks good.
www.audioholics.com
good here too
m8audio.com
Maybe some members here can lend their experience.
Are Two Subwoofers Better than One?
We discuss the benefits of multiple subwoofers by comparing measured performance of two subs vs one. We discuss using two smaller subs instead of one larger one to provide better bass for all seats.

good here too

Why Two Subwoofers Are Seriously Better Than One
Subwoofers are a great addition to a system and I personally recommend running multiple subwoofers whenever possible. Below are a few reflections and benefits of running multiple subwoofers in a system: Aesthetics can play a large role Two or more smaller subwoofers may very well outperform a...

Maybe some members here can lend their experience.