Two 12V batteries can be effectively used in parallel to double the capacity while maintaining the same voltage. This setup is ideal for applications like powering a trolling motor, RV systems, or other devices requiring extended run time without increasing voltage. To connect them, wire the positive terminals together and the negative terminals together, ensuring you maintain a 12V output with increased amp-hour capacity. This configuration allows for longer usage without depleting the batteries quickly, making it suitable for activities like fishing or camping.