Comb filtering occurs when two identical (or nearly identical) signals, one delayed in time relative to the other, are added. Depending on the delay time, the resulting summed signal can sound hollow or “boingy”, and is usually considered an undesirable sound.. Comb filtering occurs most commonly when signals are combined electronically, such as in a hard disc based recording system, but can also occur acoustically, such as a talker located slightly off axis of two identical microphones spaces inches apart.
(Thanks to Ken-at-Bose for this information).
Minimum Distance Running Two L1®s in Dual Mono
Phase cancellation can be an issue if you are running Dual Mono (two L1®s from the same mono source). This can be an issue when two or more L1®s are fed the same signal.
The minimum recommended distance for two L1®s running the same signal is 20-50 feet.
Only when fed the same signal, "Interference between line arrays would be more severe [than for non-line-array speakers]. In fact, the current Bose design rules for installing MA12 line arrays recommend ... a spacing between line arrays of between 6 and 15 meters! (20-50 feet)" The same applies to the L1. - Chris-at-Bose  (Clarifications added by the author of the quote.)