As far as smoothing out the bass response 2 subs can be better than 1, but only if you take great care in setting them up. In order to do this properly you really need some sort of tool that will give you visual feedback on how the bass is behaving such as a computer program like ETF, or a stand-alone unit like a velodyne sms-1.
Two Subs Vs. One
Trying to augment the sound from a pair of Totem Hawks in a large room (rest of system is an Olive Musica server, Eastern Electric Minimax Pre, Classe CAP-150 Amp). I am doing some in-home auditions of affordable single subs (mostly REL) and really like what they do for the sound. So far, I am leaning toward two smaller subs, one behind each speaker (ie. REL Q108E's - I like the sound better in that position and it looks better - wife factor) versus one larger one in the corner (ie. REL R-205). Can't find two demo units of the same sub to audition the two-sub option and was wondering if anyone had experience with this comparison.