Simplest possible circuit needed please

[L'eau.Life]

I have a small powercat with an engine in each hull each fitted with a 70A alternator. Until recently only the port motor had a house battery connected via a split-charge relay. This single 130AH battery serviced all the domestic 12V power and can be isolated via a simple key-type off switch
I've now installed a second 130AH deep cycle battery in the stbd hull and have purchased the relay, cables etc to charge from that motor. The intention being to power a 1kW inverter to provide 240VAC.
However; ideally I'd also like to be able to bring the second battery into play and avoid having to run a motor just to charge the house system when I've been drawing on the 12V supply whilst anchored in a nice quiet bay for a few days and/or use the first battery to give back-up to the inverter.
I'm not clever enough to understand the magic of diodes etc. but do appreciate that having the opportunity for 2 alternators to feed a single accumulator bank is probably unwise so I'm after some guidance for a simple, low cost and fail-safe solution that will achieve the above.
Any and all help will be gratefully received!
Cheers, Alan

[jonr]

Just hook the batteries and alternators in parallel with properly sized wire and fuses. Is there a concern you have with this?

[L'eau.Life]

Is there not an issue having two alternators effectively charging a common accumulator (being the two batteries in parallel) If they aren't going to fry each other then that's cool but I have a set of jumper leads with a device in the center that is supposed to stop damaging a car alternator when jumping from a running motor and the second one starts.

[TollyWally]

My jumper cables have no such device and haven't hurt anything yet, despite countless adventures over the years. Red to Red, Black to Black, double check the ground. So far so good.

[L'eau.Life]

Well OK - I did ask for simple
Thanks guys

[CDK]

Alan, the simplest possible circuit is hard-wiring everything together.
Two alternators can be wired to charge one battery, when both are running the charge current will be more or less distributed at first. When the battery voltage rises, the one with the highest regulator setting will do most of the job.

Adding a battery switch so power can be drawn from one battery only is a good idea, but the situation where both batteries are isolated from the alternators must be made impossible. That would be a reason to use diodes across the switch terminals.

[L'eau.Life]

Thanks CDK. Hard-wire it shall be!