OK, at the risk of admitting a code violation, here's a different
variation of the problem. I ran a double run of 12/2 w/g to my shop
(because I had it available vs buying new wire). But it is fed from a
single 20A breaker. The double run was for voltage drop rather than
additional current. Technically I guess that's a violation, but I
don't see a safety issue, do you?

I did the same thing when I ran wires 120' to my boat house because I could
buy 14/2 for almost nothing, but I thought the VD would be excessive.
I used one cable for the hot and one for the neutral; they can't hardly get
separated that way.

No, I don't see anything wrong with it; but mine is technically an extension
cord...