I'm building a box to provide plugin power to a stereo pair of electret mics. The electrets terminate with left channel on tip, right channel on ring, and both grounds connected to the sleeve. I found the circuit for powering a single channel online (see attached) but I'm trying to figure out how to make it work for my stereo configuration. Would it be possible for me to simply duplicate the circuit even though the mic grounds are tied together?

 

Your diagram shows Phantom Power on a Balanced audio connection. This means that both audio lines have +48V to ground but NO voltage BETWEEN the audio lines.

Because you are using stereo on a single TRS (tip, ring, sleeve) jack you cannot use phantom power because each of your signals are unbalanced. You would have to have 48VDC between signal and ground. This will be BAD! You will get a loud pop when you plug in your microphone - could wreck your speakers and damage your amplifier.

A better way will be to make a dual microphone pre-amplifier that has balanced inputs with phantom power. Take the outputs of these amplifiers to your TRS jack and amplifier. Another way (the best way, actually) is to use centre-tapped audio transformers. Good ones are expensive, though.

There many microphone pre-amplifier designs on the web - Google.