Bare bones dual 12V battery isolation circuit?
The starter neg. is usually connected to the chasis with entire body, no additional wire is present. Another propose is don't mess with starter, as there is a large amount of current and the wire has to be bolted very tight - there is enough place for just one large wire. You can therfeore take the negative for your aux. battery from the chasis .
For other things you would need a battery isolator - simplest is to use two diodes, but you need to alter the voltage of alternator output, because a diode has a voltage drop cca. 0.6 V. More sophisticated is a battery isolator using MOSFET, also know as battery combiner. Mosfets have very low internal resistance, so the output voltage of the alternator can remain as it is, but it needs some logic circuit to turn on/off mosfets.  

http://dmbruss.com/zRedRover/RR_EW_BatteryCombiner.htm