The Victron DC-DC has just one job - charge the leisure battery from the starter battery/alternator.
The Ablemail AMT12-2 takes a small amount of charge from the leisure battery (either constantly or only when it's receiving a charge from solar) to keep the starter battery topped up and at a constant 12.3v.
Some all in one units have the ability to trickle charge the starter battery but no Victron DC-DC does so if you want to keep your starter battery topped up, you'll need the Ablemail AMT12-2.
