#6 welding should be fine and it is commonly used for battery leads, jumper cables, temporary generator connections etc.
As BCK said... No problem using 6AWG welding cable, but not doubling as mentioned
A word of caution though… if planning to run parallel feeds you need to replace your existing cable as well. You will run into other problems if using two different cable types. Parallel cables must be exactly the same length & type.
I would recommend going with either 4 or 2AWG cables (look on the internet), if you want to increase the wire size. The problem with running 2 cables of 6AWG is that even though you may be able to get them the same length, the crimped lugs (only use compression lugs, no hammered lug tools or screw fittings, you could be asking for problems in the future) may not be compressed exactly the same, both of which will produce a different resistance for each run.. As time goes on and the ends get a little oxidized, that will also alter the resistance of each connector. This would add to the voltage drop you are trying to avoid
As I mentioned above, thin welding cable should only be crimped with a hydraulic crimping tool and compression fittings (lugs). The reason is unlike the thicker stranded wires, the thinner fine wires react more to temperature differences (expand/contract). So when they contract they are not as tight in the lug, a problem with screw in connections or connections that are not compressed with hydraulic crimping tool. Most of these tools create somewhere around 8 to 10 tons of pressure, which makes for a nice snug fit. Try applying that much pressure with a screw in or hammer compressing tool.
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