If you're going to make extensions for stereo headsets, you'll need at least three conductors in the shielded cable. One conductor for the mic line, and one each for left and right earpieces, and of course, the shield itself will be the common ground for all. The earpiece wires really don't need shielding, but the microphone line does. Some techies frown upon having earphone/speaker-level audio wires running parallel to a microphone wire inside the same shield for long distances due to the possibility of cross-talk bleeding over from the headphone wires to the mic wire inside the same cable, but I've made aircraft headphone extension cords before with three-conductor shielded cable several feet (6-8) long and haven't experienced any problems with bleedover or parasitic induced oscillation (aka feedback squeal) from such an extension cable yet.
Now, if you're running permanent cables, installed into the aircraft, from say... a front panel-mounted intercom system, all the way to jacks mounted in the back for rear seat passengers, you really need to use separate shielded cables for the mic and headphone jacks, and the cables would need to be Tefzel-insulated aircraft wire.