I am using miniature lavalier omnidirectional body mics taped to actors' faces in a theatrical application. Whenever I have two or more actors speaking in close proximity to each other, I am getting a very unnatural sound from their mics that is perhaps caused by comb filtering. The highs predominate, and it sounds like they are talking into a coffee can. When they move away from each other, it sounds fine again. Other than riding the faders to keep only the speaking actor's mic on, do you have any other suggestions?
Reply: You are definitely getting comb filtering from phase interference between the two mics. Also, since you said "the highs predominate", it sounds like the low frequencies are cancelling, which indicates a polarity difference between mics. In the XLR cable coming from the wireless receiver, reverse the connections to pins 2 and 3 and see if that helps. Or flip the polarity switch on one mic channel if your mixer has polarity switches.
To reduce comb filtering, put the mics as close as possible to the actors' mouths. Another possibility is to keep the mics farther from the mouth, but use only one mic to pick up both actors when they are very close together. Otherwise you'll have to ride gain as you mentioned. An unusual solution is to pan the two mics slightly left and right (if you have a stereo PA system).
Because of their better isolation, cardioid lavs might have less of this problem than omni lavs. However, cardioids have more handling noise and breath noise.
If your two lav mics are one foot apart, a 1 msec delay or more in one channel might help