You can do 2 runs of 8 awg from your battery. There is nothing physically wrong with it. However, it is ghetto. Also It may be more difficult to run 2 sets of 8 instead of 1 run of 4awg. Remember, you will need to fuse both wires close to the battery. So a single run of 4 awg will be one less fuse.

Ideally you should do a single run of 4 (or larger so you'll be set for future expansion). But you don't have to. Your suggestion will work.

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