I'm pretty sure Dave has a second battery setup similar to how you want..... interested to see what he has to say.

Otherwise what you're doing is very possible/logical. Pretty sure every boat is setup like that.

I would recommend saving some trouble and not trying to wire it so both batteries are capable of starting the vehicle. The simple way someone would think of doing this is just running some jumper wires on a disconnect switch so the second battery would "help" the primary battery start the vehicle. However this usually doesn't work. If your primary battery dies its usually better to swap the other battery in it's place since the combination of bad+good battery = bad battery (hence why its hard sometimes to jump start vehicles with really dead batteries.