Messing with boats I've come accross this a lot.
You can do it with a simple relay, tho it has to be a heavy duty one.
Connect the contacts of the relay so that when its energised it connects the 2 batteries together, then connect one side of the relay coil to earth, and connect the other side of the coil to the alty warning light terminal, when the alty starts charging the relay will close and connect both batteries together, the only drawback with this is that if the engine battery is fully charged and the other battery fully flat, when the relay closes there will be a big inrush current into the discharged battery, that said you come accross the same circuits in boats and caravans a lot.
When you start the engine you have to just blip the throttle once to make the charge light go out, as the alty has to energise the relay.
Theres plent of info on the net how to connect it all up, google 'split charge circuit'.
If you want to spend money, stirling do a proper split charge controller electronic module, which charges and conditions both batteries.