<<The reason is that should one cable go bad for whatever reason, the other conductor will be undersized for the load and will be subject to overheating.>>
Assuming each has a breaker that is less than the wire's max amps, that is not an issue.
I've been playing around with circuit diagrams.
I will agree I don't like the idea of switching the use of the wire between House and Winch. Too many complications, particularly with positioning switches and circuit breakers and what happens when one is open.
So I won't be doing that....
If we are not adding a winch but just doubling the House is not a problem.
The point is not to increase capacity but to reduce the voltage drop felt at the panel.
Keeping the same 50 Amp circuit breaker, there is no danger of one wire being undersized if the other is damaged or open....in fact it prevents it.
The existing stranded wire is in fact a bundle of individual wires in parallel.
If caution is taken to tape the ends together to identify them as one, and connect them to the same terminal at each end, then they will act as one larger cable
If you have ever used the "Both" position on a battery switch, you have put wires in parallel.
<<You can't assume either that each conductor will carry half the load either.>>
This is not an issue. The current will divide based on the resistance of the wires. The total resistance of the wires in parallel will be less than the lowest of them individually.
This is what we are after because there will be less voltage dropped across the wire as a percentage of the voltage at our panel.
But of course, I went off topic because this is about feeding the winch/capstan...SORRY!
I'll have to measure the actual distance on the 265. I'd like to use 6/2 duplex if I can rather than separate red/black 4awg. I don't think they make 4/2