In article , Bill wrote:
I shouldn't have the least worry about threading a 12-2 romex cable
through 3/4" holes, and back through the same holes, should I?

Not at all. That's plenty of room. 5/8 is probably enough.

Nothing else will be in the holes. The issue that bothered me (a
little) is knowing that both cables would always be used at the same
time.

That won't matter, it really won't. Pros do that all the time.

I'm going to try out my new auger bits tonight--starting off by
making some bit props to get the holes lined up..should be fun. : )

Don't spend too much time getting the holes lined up exactly. The cable is
flexible enough to tolerate even a fairly gross misalignment, and once the
drywall is up, nobody but you will ever know.