How do you connect two 12 volt batteries and keep 12 volt system?

To connect two 12-volt batteries while maintaining a 12-volt system, connect the positive terminal of the first battery to the positive terminal of the second battery, and then connect the negative terminal of the first battery to the negative terminal of the second battery. This setup keeps the batteries in parallel, preserving the 12-volt output while increasing the overall capacity (amp-hours) of the system. Ensure that both batteries are of the same type and charge level for optimal performance.