The general idea is that each sub interacts with the room differently and produces a significantly different in-room peak-and-dip pattern, and it is the SUM of the two dissimilar peak-and-dip patterns that matters.
Riiiiight..... so...
Nothing against the strategy Erik recommends, his may work better; either one will result in a worthwhile improvement.
What exactly do you, @audiokinesis ,think the difference is, in setting up 2 speakers between what we are recommending?? , because I can't see it. :)