We used a battery isolator (BI) to feed 12v to switches we had connected to the Renogy's LC and D+ terminals (switchable off, 20a, & 40a modes). The BI would only pass power to the switches when the primary input voltage to the dc to dc charger was 13.3v or greater (alternator active/online). Prevents a dc2dc from discharging the batteries when the engine is not running (alternator offline <13.3V) even if the D+ switch (and/or ignition) is left inadvertently ON. Mounted the BI right next to the dc to dc charger using 3 short wires. No need to run wires to the ignition. Especially handy option for those folks with a dc2dc a good distance away from the vehicle battery or ignition switch (i.e., travel trailer, truck camper, etc.).
Not to go off on too much of a tangent . . . we installed a pair of 40a Renogy dc to dc chargers in parallel and used a few switches to control 12v input to the LC and D+ terminals on each dc to dc charger. This allowed us to select 0a (off), 20a, 40a, 60a, and 80a of charge current. A simple, easy mod that provides a lot of operational flexibility. Of course, wouldn't recommend trying this unless you have a very high-output or dual alternator(s) (>250a).
Although I haven't personally tried it, it may also be possible to parallel a pair of 40a and 60a Renogys and have the option to select 0a (off), 20a, 30a, 40a, 50a, 60a, 70a, 80a and 100a of charge current.