Two mono microphones on a single stereo input jack?
What you want to do should work, as long as your sound card really has stereo microphone inputs.  Most will have stereo "line" inputs and at least some kind of microcphone input, but you need to check whether that mic input is stereo or not.  If it is, just hook up the two mics to the common ground and the other side of each mic to one of the two input channels.  Make sure to use shielded audio cable.

If you don't have a stereo mic input, then it would be better to use a separate preamp so that each signal is amplified to "line" level, then put the two line level signals into the line level input of the sound card.  In that case you have to be very careful about ground loops since the preamp will be separately powered.  Plug it in as close as possible to the same place the PC is plugged in to, and don't hook up a separate deliberate ground to it other than thru the line in connection of the sound card.