So here's a dumb electrical question

I see nothing wrong with "the idea" of this depending on circumstances. An alternator charge wire is a good example, as most automotive outlets don't carry no6 wire. Therefore, paralleling no 12 or 10 makes sense.

This might also be a great trick in an emergency, somebody burns a battery cable on the headers etc.

I think you get enough no 12's in there for starting you will have a REALLY big cable.