I've just done a diagram on Eagle for you...
Note:
The relay, 2 x fuses and switch are optional but with the switch in the ON position it will lock both batteries together as one when the ignition is on.
With the switch on - This allows either alternator to charge both batteries.
With the switch off - This disconnects both batteries which are now independent
With the ignition OFF the 2nd battery will always be disconnected from battery 1 so that you can draw power from it whilst always being able to start the car from the first battery.
Blue wiring = existing wiring
Green wiring = new wiring
R2 is also optional BUT without it the alternator will stop charging if the bulb blows.
The fuses (in either version) should be 50-60amp fuses.
Here is the alternative SINGLE alternator DUAL battery version (split charge).
Note that the switch now allows the 2nd battery to be charged so it's default position is ON. When the ignition is off the batteries are disconnected from each other and you can draw power from it without flattening the main battery. The idea is that you connect your lights, stereo, CB, cigarette lighter etc to the 2nd battery.