When using redundant microphones, such as dual lavaliers for news announcers or two podium mics for a politician's speech, mix in only one or the other, but not both.
If both mics are mixed together, there will be phase cancellation due to the path length difference between the speaker and each mic. Some frequencies will be in phase and add together, some will be out of phase by some degree and partially add together, while others will be totally out of phase and cancel each other. This is often referred to as comb filtering. Electronic equalization cannot make up for the acoustic cancellation.
The voice may sound tinny, or lacking in bass. If cancellation takes place in the 2 kHz octave band, speech intelligibility will greatly suffer. Timbre of the voice can even change as the speaker moves his/her head relative to the microphones.
So use one of the mics but have the backup mic ready to be quickly switched in should the need arise.
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