A common ground shared between two mics is not a problem.
My "standard" microphone cable has 5 conductors and I run stereo mics or two separate mics through them all the time. I have never had a problem with a common ground.
Every one of my stereo mics has a common ground between channels. In fact, I do not know of a stereo mic that does NOT use a common ground. (However, I am sure someone will come up with an example to prove me wrong.)