All things equal, more subs equals more efficiency and output capability. Smaller rooms have a problem both absorbing bass energy, however, for proper RT60, and long bass frequencies don't support well in diminutive dimensions. So I like small single subs in small spaces, multiple 8's placed together or single 10" sealed woofers in medium spaces, and maybe 12" dual's or better stacked together, or flanking each channel as stereo subs in large spaces.
Problems you run into with spacing multiple (in this case, two???) subs, spaced 1/2 way points along the wall boundaries: PHASE CANCELATION at different seating locations! Sit closer to one sub vs other in a room with subs run in mono, and you'll have a good ol time trying to not get phase cancelation issues between the two! It's simple acoustics.
Pick the sealed subs in small and medium sized rooms, and go multiple subs for more output and dynamic range from full on HT duty applications! ..but keep em together, say, in center of front of the room, near the main center for best crossover, easier integration, and lack of phase issues from seating locations. Blends best, I tell ya.
