Questions on installing an intercom

I have an electrical/audio question. The intercom I'm installing has a
wiring diagram that shows the sleeve terminal of the phones and mic
connected to the shield at the radio end. This seems pretty typical.
My question: is it okay practice to use the wiring shield as one of
the conductors? In other words, can I use 2-conductor shielded wire
for stereo headphones (left, right, and shield for ground) or should I
buy 3-conductor so the sleeve terminal has its own wire inside the
shield? I ask because both the sleeve and the shield get tied to
ground at the radio end anyway.

Also, should I use a separate wire for the mic audio, or can all of
the conductors run inside the same shielded wire using a common
ground?

Thanks,

Craig.