You would do much better to determine why there is the voltage drop. Many things can cause this.
If you add a second smaller wire, it will need separate fuse protection for the wire ampacity, preferable with a T type fuse with large currents. The problem with a second smaller wire, if the load is high and you have a connection with resistance on the larger wire, 2 things can occur. One, the connection gets hot enough to cause a "up in smoke situation". Second, if there is enough resistance on the larger wire, current will flow more on the smaller wire causing a possible "up in smoke" situation. Even worse if the larger wire was to go open circuit.
The correct way to measure voltage drop on any point on a DC system is to have the positive multimeter lead at the source or at a location on the upside of current flow, then use the other lead downstream to read the voltage. This method creates a parallel path to read voltage potential. If the total of the entire circuit has over 0.5V it indicates high resistance and needs to be addressed.
You may find that crimps and connections are the cause of the voltage drop. You did not state the current system voltage and the amp draw. Those would be helpful to determine if 1/0 cable is sufficient to handle the load. If you had done the calculations before building the system and 1/0 was sufficient for the required amp draw, then you should start checking for resistance in the circuit.
