First, when 2 signals arrive at the same place-with the same signal, at different times-this is textbook combfiltering.
I'm not normally one to argue with Ivan's extensive system tuning expertise, however, he's left out a very important point about "textbook combfiltering" - level. When two signals arrive at the same place with the same signal at different times with similar levels, you get textbook combfiltering. With delay speakers, the levels are usually more than 9dB higher than the mains - making any combfiltering artifacts inaudible. In practice, things are often more complicated, and I bow to Ivan (who's had a LOT more experience than I have in this area).