I have used two subs in my own setup, as well as for freinds locations as well, and can wholeheartedly recommend using more than one to fulfill that beast inside us that says you can never have enough bass! I can however, guarantee, based on my experiences with myself and friends, that one should purchase a second bass unit, i.e. sub, only if it goes lower than the original sub in frequency response. I have tried numerous subs from Klipsch, Velodyne, REL, Mirage, Yamaha, JBL etc. and have never found that two of the same kind make more bass!! It's only by extending the depth of the bass that you will feel the difference.
Do try this technique, and let us know what you think/find out!
Maketh bass so that the doors rattle, the windows fall off, and the neighbours move out!!
Two subs are better than one, and even better than none!