OK, here's the technical correct thing to do, which is not what some do, and you'll probably hear from them too.
Never hardwire anything in parallel. If one of the two batteries has a problem, it WILL negatively impact the other battery. There are many cases of people with 2 main batteries but still stranded because both are too low to start the outboard. As one battery dies, it drags down the other battery. Result: TWO useless batteries.
Get a battery switch. I can point you to a good one available on an auction site for $30. The two batteries get wired to this, and typically you select #1. IF, some day, #1 goes south, you move the switch to #2, and boom, it's like you never had a battery issue. Finish your day, then address replacing battery #1.
Charging: Best answer, get another 2 bank charger. Next choice: get a 3 bank, and connect bank #3 to the common connection on the switch. Then you can charge battery #1 or battery #2 by changing the battery switch. This choice works fine especially if you normally don't need to use battery #2. (That battery would just need periodic top-off charge to keep it ready as your back-up.) The switch is an advantage that you can also turn off all electrical if you need to.