There are so many ways to set up a dual battery system. But all systems have disadvantages.

A relay is cheap and simple but if one of your batteries is empty this system will cause a lot off current being pulled from the better battery. This in the long run can damage the health of your batteries.

A Diode bridge prevents this. But a standard bridge has a voltage drop of 0.7 Volts. This means that the battery will never reach its full charge voltage unless you modify the voltage regulator of the Alternator.

Victron Energy ARGO FET is a splitter that uses mosfet technology. Although they are more expensive they only have a 0.02 Volt drop. The space next to the reservoir for the wiper fluid was perfect for this ( some people have the battery isolator switch there)