Phantom power is still done much like Ron described, but having two microphones there won't cause problems with the phantom powering
circuit. It may lower the
voltage at the microphones, which some may be sensitive to, but most these days are designed to work okay at lower voltages since a few devices/systems put out lower voltages--mainly battery powered
portable stuff; practically anything resembling a "normal"
mixer will use 48V. If the microphones in question are
dynamic microphones, it makes no difference at all as the
phantom power is completely unused (that's basically the phantom part of it).
The main drawbacks I can see are potentially lower audio quality (higher noise, more
distortion sooner, lower levels from any given
microphone,
etc.) and, probably most importantly, a huge reduction in flexibility when operating the
system. You can't balance between the two microphones. You can't pan them a
bit to make a stereo recording. You can't independently adjust their equalization if desired.
Low source impedances to a
microphone preamplifier won't cause any problems for the preamp; there's no problem with shorting the inputs, and indeed that's a standard way to measure aspects of their performance. Audio generated by a
dynamic microphone from a signal
level such as another
microphone would generate is essentially none; the
power just isn't there to move any air. A
condenser microphone would probably not produce even any theoretical audio output from a signal superimposed on its output due to their built-in preamplifier circuitry. Worries about
phase cancelation
etc. at the
mixer are pretty much moot because you'd get exactly the same
phase cancellations in the process of mixing the
microphone signals together if they were separated.
