Wiring two 12-volt batteries together depends on the application it needs to be used in. If double the voltage from 12 to 24 volts is needed , the batteries are wired in series (negative, positive wired to negative, positive). If 12 volts are needed, the batteries are wired in parallel (positive wired to positive and negative wired to negative).
Start with the first battery. Attach a wire to the negative pole. This will be the negative lead. Next, wire the positive pole on the first battery to the second battery's negative pole. Attach a wire to the second battery's positive pole for the positive lead. This will give 24 volts with the same amperage. Attach the leads to the desired load.
When wiring two 12-volt batteries in parallel, simply wire the positive pole of the first battery to the positive pole of the second battery, and then attach the positive lead. Repeat the procedure for the negative side, wiring negative to negative and then adding the negative lead. The result will be 12 volts, but the amperage will be doubled. The leads are attached to the load as noted above.
There are some important safety considerations you need to keep in mind when wiring batteries together. Always use the same volt/amp combination when wiring batteries together. Do not mix and match. When wiring the batteries, never let the leads touch each other. This can cause a short and may cause overload and excessive damage. Also, make sure to understand the voltage and amperage requirements of the load before selecting either a series or parallel-wiring scheme.