Stacking sub-woofers

The consensus seems to favor two subs vs. one. Two subs give less distortion, more realistic sound, less work for each sub to do and more. Can subs be stacked? Is there an ideal spot to place two subs figuring the "average" rectangular room?
Separating subs can cause cancellation of the bass waves. Stacking the subs works much like the stacked mids in a d' appolito speakers design. Wide dispersion horizontally and narrow dispersion vertically. This is the way the subs are arranged in a IMAX theater. The sub system is mono just below the center channel.
Stacking subs will not produce interference patterns that effect dispersion—the wavelengths are too long. Stacking them won't help smooth out the room response. See Floyd Toole's book "Sound Reproduction: The acoustics and psychoacoustics of loudspeakers and rooms."
It's generally a bad idea that undoes (virtually) all of the benefits of using 2 subs - that idea being that using 2 subs will create 2 different (hopefully offsetting) patterns of reinforcing/destructuve interaction between each sub and the room.

As to optimal placement, there is no formula - as each room differs. IME, the easiest way BY FAR to optimize placement is to use a Real Time Analyzer. Room Wizzard is one choice and the Velodyne SMS-1 (which also provides a parametric EQ section to further fix the bass response) is another. You just move the subs and look at the response pattern on screen. Very easy, very effective.

Good Luck,