Dual Castles non-colocated, (per your original post), will give you 3 db more output under ideal conditions. Three db is not that great a difference. And, yes, two can smooth your FR some. Question is, have you measured your current FR and how much smoothing does it need if any?
Also, seems to me that adding a second sub is a hit-or-miss way to EQ your bass.
I think that until you can put into words what you think you need to satisfy yourself for the long run, you might want to sit tight. Do you want the ability to reproduce 10 Hz? Folks with the Conquest can achieve that. Remember, the Conquest did 103.8 db at 12.5 Hz in a 7,500 cubic foot room. Or, do you just want more of the same as you have now?
Right now, a number of folks who had been satisfied with ported subs are selling them and going with sealed subs for improved sound quality. The Ascend/Rythmik, the HSU ULS-15, and the J L Audio subs come to mind as having better sound quality than most ported subs, while also being pretty powerful.