I have one 240v/20a circuit in my shop and it looks like I need another.
If I ran them both to a subpanel, I would have 40a! That should be rather
more versatile than 2 20a circuits. Doing it properly with #8 cable is much
more difficult (I just ran 8/3 from my generator to a transfer switch, and
it wasn't fun) and much more expensive as I already have nearly free 12/3.

I know you cannot run a circuit over multiple wires because if one breaks
you have an overload on the other, but each cable would be on its own
properly sized breaker, so that can't happen.

I probably won't be doing it because it is too much trouble for something
that will kill my resale value, but I don't see anything wrong with it.
What am I overlooking? All I can think of is that if one conductor fell off
there would be an unbalanced load between the two cables, but that could be
made moot by running both through all the same holes.

If I was smart I would have put a subpanel in the first time. If I was
really smart I would have figured out how to make the subpanel work as my
generator hook up; but those boats have sailed.