If you want to run a wire direct from the alternator output to the ammeter, attach it to the same side as the main splice.
If keeping the original wires, then there are two parallel lines (as shown below). So the load is shared.
If not keeping the original run through the bulkhead, check the size of the wire from the ammeter to main splice. Might be worth taking the unused/ damaged wire out of the bulkead and putting a ring terminal on it. So then there is new heavy (8 or 10 gage wire) to terminal stud, and two older 12 gage wires to the main splice.