Attach the positive terminals together and the negative terminals together. Following this example where there are two 12v 200ah batteries connected in parallel we will therefore have a voltage of 12v volts and a total capacity of 400ah ampere hour.

Rv 12v Information Everything You Need To Know
Connecting two 12 volt batteries in parallel. The parallel connection of two identical batteries allows to get twice the capacity of the individual batteries keeping the same rated voltage. But what happens if you wire batteries of different voltages and amp hour capacities together in parallel. Take another set of batteries and wire them together in parallel as well. Connecting them in series will seriously damage the inverter. If the input rating on the inverter no matter what the output wattage says 12 volts you must only connect two batteries in parallel. Connecting batteries of different voltages in parallel.
Finally take a jumper wire and bridge the positive terminal of one parallel connection to the negative terminal of the second parallel battery pair. This is the big no go area. Two 12 volt batteries in series give you 24 volts. Two 12 volt batteries wired in parallel continue to produce 12 volts but the capacity doubles. Make a parallel connection for the two 12 volt batteries. Two 12 volt batteries in parallel give you 12 volts.
Four 12 volt 2000 mah wired in parallel can provide 12 volt 8000 mah 2000 mah x 4. Charging 12 volt batteries wired in parallel requires a 12 volt battery charger. Connect the two positive terminals on both batteries together. Take two batteries and connect them in parallel. Batteries may be wired in parallel or in series. Use one battery cable.
Two 12 volt batteries wired in series produce 24 volts but the capacity remains the same as one for a 12 volt battery.