If I connect two start batteries in parallel will the outboard charge both simultaneously or is a switch needed.

It will charge both batteries, if they are perfectly matched then the charging voltage will be 1/2 of normal charge to each battery.

Would think no, think the charging system on the motor is designed for twelve volt not 24.

Would think no, think the charging system on the motor is designed for twelve volt not 24.

"parallel".... better off with perko battery switch.

When you 1st connect two partially of fully discharged batteries in parallel, the less discharged battery will discharge into the more discharged battery....this will have at least some negative effect on battery life.

When you connect the motor or charger, both will receive the same applied voltage, but each will accept and receive differing amounts of charge current until they both reach full charge.

When the motor is running, it will charge both to its amperage ability which varies with RPM. If you were to use a single bank charger that uses charge lead temperature sensing to compensate and optimize it's charging voltages, this feature and safety benefit will be lost for the jumpered battery not connected to the charging lead.

Parallel charging works but is a compromise. Best to avoid repeated parallel charging especially if the batteries are not of the same age, size and condition or not always parallel discharged. A weak unusable battery will be harder to detect.

A separate house and cranking battery usually offers several advantages over a two paralleled cranking battery setup. Most better and larger boats throughut the world use a dedicated cranking battery + separate house electronics battery setup. In that case a combine switch is usually included to add the batteries together in a cranking emergency.

Thanks all, appreciate the info.

A couple of things. Someone said you voltage will be cut in half. No, you current will be cut if half. Your batteries will charge but not as fast.

Another thing. Most charging units get ruined because people throw an undercharged battery in their boat and expect the system to bring it up. It will but when your battery is low, your charging system is going to supply more current than normal charging. This extra heat is generally what kills a charging system. Best practice is to keep your batteries charged up at home. Much better for your system.

Wow, I did make a mistake ...current is correct ...my bad ...back to my doghouse

No worries heybaylor... happens to the best of us.

I have a perko switch on my boat, OFF, 1, ALL and 2, I was wondering if I set the switch to ALL and put a charger on one of
the batteries, will both batteries charge?
Or do I just leave the switch in the off position and charge the batteries individually?
I'm just talking basically a trickle charger..
Thanks

Two 12V batteries in parallel is still 12V, not 24V.