Two microphones connected together and driving one another. Although the voltage is small, each microphone is now also acting like a speaker. Although techinically correct, it doesn't matter.
Nobody cares about that.
If the extra mic makes your life easier, plug into the parallel universe and widen your pickup pattern. Abdul has done this many times, even with condensor mics. The big downside is lack of control over the individual inputs. Two microphones connected together and driving one another. Although the voltage is small, each microphone is now also acting like a speaker.