Yes, that will work OK, but is not optimum, because when you combine the house bank and the start battery it requires you to physically turn the switch to "BOTH", and it then electrically links all the batteries together. Depending on the state of charge (SOC) of the batteries, this can cause an inrush of current
to the relatively depleted batteries, and a corresponding outrush of current
from the battery(ies) with the higher voltage.
This may or may not cause a problem downstream.
The preferred way to deal with a house bank and a separate starter battery these days is to use a voltage follower device like an EchoCharge or a DuoCharge. This little device sits between the house batteries and the start battery. When it senses a charge on the house batteries, it bleeds off some of the charging
current to top off the start battery. In implementing this strategy, you'd want to connect all onboard charging devices (alternator, battery charger, solar panels, etc.) to the house battery bank.
The logic here is that the start battery requires very little charging
. Typically, to start a small to medium-sized diesel
requires a lot of current for a few seconds, resulting in a usage of only about 1 amp hour from the start battery. This is replaced in minutes by the alternator
and, unless you have fans or other devices connected to the start battery it thereafter requires only a small maintenance-level charge.
If you choose to rearrange your charging systems in this manner, be sure to include a properly sized fuse on the alternator
output (in your case, a 50A ANL fuse located near the house batteries).