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