My suggestion would be deliberately placing both subs with asymmetry in mind. For instance, one might be on the front wall near the left corner, and the other might be on the right wall near back corner, but a different distance from that corner than the first sub is from its corner. (You want to avoid running that second sub - the one furthest from the mains - up very high in frequency, because you don’t want it passing upper bass energy loud enough to betray its location when the music is playing.)
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. Nothing against the strategy Erik recommends, his may work better; either one will result in a worthwhile improvement.
If both subs have phase controls, there may be yet another strategy you might try.
Whatever placement strategy you use, imo the exact position of each sub is less critical than when you only have one sub.
Duke
