The second run of wire allows more amperage/current to pass from one point to the other.
Its called ohm's law and is a simple triangulation between volts=amps=watts. 12,000 watts/12 volts= 1000 amps. @14.4 volts you get 833 amps being drawn.
As you see more of one unit directly results in less of the other. Same thing that occurs in an amplifier when presented an 8 ohm load instead of a 4 ohm. half the ohms, twice the current available on the same voltage!
Now we go to a wire gauge rating chart. 1/0 ga. is rated for 250 amps up to 12' distance. Being you have 2 batteries and the one is hooked directly to the amplifiers, you can cut the amp draw in half. So if your run is 12' or under you should be perfect with a second run of 1/0.
If you were competing you would want the alternator to produce the 833 amps @14.4v, and would need wire rated to pass that amount from alternator all the way back. For a daily this is not necessary. Let me put it this way, if your alt is only rated at 250 amp's there is no need to run a bigger than 1/0 wire from it to the battery(s). What you are trying to accomplish is move the alternator + front battery current to the rear.