Solar with dual battery setup - keeping both charged?
If you trickle charge the starter battery using voltage from the panels rather than the MPPT controller output you have ample voltage to add a blocking diode and prevent backflow. The current drain is low enough to not significantly affect how the MPPT controller functions. 

Long term trickle to a lead acid battery may cause damage. A simple voltage regulator set to a suitable "float" voltage is probably all that's needed to prevent this. This can be cheap simple and compact. 

You could join the batteries with a "switch" for charging purposes, with the switch being activated only when the panels were providing energy. The switch could be a transistor but in this case a relay is probably an easy and good solution. Drive relay from panel with a "controller" that senses panel voltage - or use MPPT output or signal to switch relay.

Joining the batteries via a diode prevents back-feed but if the main battery is properly managed by the controller then the lower voltage on the other battery caused by the diode drop  can noticeably affect battery lifetime. [I've seen this in practice]. Using a Schottky diode reduces the drop. The diode may try and handle starting currents in some cases so a little thought may be needed.