Toller wrote:
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?

How bout this:

Someone comes along to fix or remodel something. They see two wires
coming off the same breaker. That is normally just two runs of wire off
the same circuit, and is done all the time. They shut the breaker off,
disconnect one set of wires, and turn the breaker back on. Poof, they
are holding a live pair of wires, because the two are connected
downstream (in violation of code and all normal practices).

The same problem is even worse if there were two separate breakers,
even if they are tied.

Electricians and repairmen simply do not (and should not have to)
expect conductors to be connected downcircuit, because parallel
conductors are forbidden (with the exception of the much larger wires
that *are* allowed to parallel).

All this is in addition to the possibility for inductive heating
(especially when you are using romex, people are not used to worrying
about that), and different length cables causing an imbalance.