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?
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. 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. : )

Bill