why do you need two subs? any sound the sub generates is pretty much unidirectional. read this it makes sense:
http://val.dorta.com/audio/archives/000109.html
it may make sense when you *read* it, but when you *listen*, then it no longer makes any sense!
even if you cross over as low as 50hz, unless the sub is *exactly* centered between the speakers, you *will* be able to localize it. and, w/this location, the best-case scenario will be no change in soundstaging. if you have two subs, placed flanking your mains, the soundstaging will actually improve.
others' comments about less room interaction difficulties, & more spl/less distortion definitely apply w/two subs vs one sub.
ymmv,
doug s.