Multiple power cables to fuse box?

My current setup engine bay forward is pretty well set-up, howver there is only a single (at a guess), 10 AWG cable to fuse box. This is all loomed together, taped and wrapped with the ignition and fan cables so to replace that cable I'd need to replace the others (The wrapping is on tight!).

Can I run a second 10AWG Cable to the fuse box (so there would be 2 x 10 AWG Cables running in)? Am I right that this would give me more capacity at the fuse box without having to replace the 10AWG with a 6 as an example?

I would say no. If you were to go to like a 4 gauge wire then yes you could supply way more power. Doesn't matter how many 10 gauge wires you have they all can only handle so many amps. Also, what is your fuse box even rated to amp wise? In the car audio world they call it upgrading the big three. Positive from the battery to the alternator and the battery ground to the frame. The third is getting a higher amp alternator. Generally you double the size of the wire so if it's all factory 6 or 8 gauge you would go up to 2 or 4 gauge. Oh, and length of your wire also hinders it amp load capability. The longer the wire the less of a load it can handle. Hope this helps.

Parallel conductors is a common practice. That said I wouldn't recommend doing so for this application since the wires would be hard to run in true parallel and be subject to different conditions. Parallel wires should be run exactly the same and as close as possible in length. They also need to be the same type (txl vs tefzel for example), bundling, ambient temp, ohms per foot, all of these differences can contribute to different resistance. Different resistance means the current they carry will be different.

Going up in size is the better call.

Thanks!

I also could be dealing with overkill here! 10AWG may be plenty given this is a carb'd 1960's truck! I'll do some maths!

Alternator is a 1 Wire Alternator and install instuctions state a 10AWG 'hot wire', i'm more concerned about the Fan / MSD that cables wil need cutting and replacing (And was trying to avoid!)

On that, how can a 100amp Alternator only need a 10 AWG Wire?

This model (https://www.proformparts.com/product-detail/66448.1F/ford-alternator-black-crinkle-100-amp-1-wire-v-belt-pulley-100-new)

Install instructions state 10AWG wire, but that's only rated for 30amps? (https://www.proformparts.com/images/ePIM/original/PROFORM_1-Wire_Alternator_Instructions_66445series.pdf)

Clearly I'm missing something!