I found dual sub use with a two-channel set up to be more trial and see what works as opposed to a specific formula. So much depends on the equipment and room set up.
I'm also using a pair of Martin-Logan subs to augment my DynAudio Contours (which already have strong bass response). I wanted full effect on the very low end but I didn't want them to over play low end bass lines. After several attempts and some advice from Get Better Sound; I have them set up just in front of each speaker, laying on their side (they are bottom firing) so that they fire at the wall behind the speakers (set up for 13' x 11' room). The cross-over is at about 38-40Htz, phase 0 and volume is only at 1/4.
Basically, 80% of the time I don't even know they are there. They don't color the music be it Rock, Jazz or Classical until a bass drum or deep bass line is emphasized and then the bass sound is tangible. You can feel it without distortion; like a live performance. On something classical like the 1812 Overture; the cannons are a down right scary rumble. For an acoustic trio the bass line sounds like its in the room.
But that was the goal only wanted to capture the ultra low that the speakers couldn't reproduce. Also noted I set my system to direct without EQ or gain on bass or treble.