split diodes or too light cabling from the alernator to the battery present a voltage drop, this drop will keep the second battery from fully charging.
most car alternators reduce the charge voltage and current when they get hot, to something as low as 13.8vdc,,, if you have a 0.7volt drop via a diode then at best you have 13.2 vdc to the battery, if the cable is small you could have as much or more loss.. then you don't have enough to charge at all.
large cable and a switch or a relay that drops out when the engine is not running, even then you will want to check to see just how much voltage you get to the remote battery, so see if it will charge in a reasonable length of time, or at all.
if your alternator has a remote sense wire, you could connect it via the relay (second set of contacts) to the rear battery, this would tell the state of voltage at the remote battery and keep the alternator up to sufficient voltage to recharge effectively.
just something to be aware of, probably a good idea whatever you do is to be sure and put a meter on the remote battery to confirm that it is getting enough juice to charge it properly.
bob g