The problems ARE more complicated when keeping multiple batteries connected in parallel. Even if he gets the batteries load tested and they both pass the test, the two of them aren't exactly equal. One will draw the other down to some extent if they are placed in parallel and left that way. Once the poorer battery gets worse, it will drain the good battery even more rapidly and more deeply. One day he will eventually have two batteries, neither of which will get his engine started to get home.

If one battery is bad and he replaces it, again the two are not equal. The older battery will degrade the new battery, and the problem cycle eventually happens again in the future. The newer battery will die sooner because it too will eventually be degraded by the remaining older battery.

By all means do as you wish. I'll keep my properly configured electrical system in place and add another year to my ~37 year streak of never being stranded on the water. I do carry tools and jumper cables just in case, but I always seem to be using them to help "that other guy" who is stranded.

If Battery 1 gives you trouble, you can switch to BAT 2 for a fresh fully charged backup battery to start your outboard, then switch back to BAT 1 to charge it. Your BAT switch must have a "combine" feature so that it combines both batteries momentarily as you switch between batteries.

you can charge dual batteries from one charger link while both are connected, it results in half the amperage going to each (more or less) and takes twice as long to charge if not longer due to very low battery.

get ya a perko switch like above, hook each to it as instructed in directions, connect your charger leg via a 30 amp rocker switch dpdt (double pole double throw, on off on)

Each of the above posts gave excellent information...... except perhaps for the quoted notion that after combining 2 batteries in an emergency cranking situation it should be followed by switching back to one battery while the alternator is charging both.

Doing so is a direct warranty violation of the more modern high charging amp capable engines as it can too abruptly alter the load on the output power driver circuit of the engine's internal voltage regulator, even if a make-before-break battery switch is used.

If emergency starting an engine in the switch's Combine position, best to keep batteries combined until engine and its alternator are stopped.