...it's the concensus that two subs are indeed better, but mostly because you will get a smoother LF response. Imaging will not suffer at all using only one sub.
I don't have any major problems (or at least I don't think I do!) using my single Strata III, and the thought of spending another grand to get what might be a slightly smoother sound isn't all that appealing to me at the moment :^)
As far as connections go, mine is hooked-up at the amp's binding posts... R + and -, and L +. I'm sure whatever sub you do will have instructions.
Twin subs might be better yet with imaging, but I'm quite happy - as a matter of fact, my sub might be the most dramatic improvement to my stereo (after the main speakers, and on most software).