I have the echo charge and my smaller start batt is always topped up... by the big brother house bank.
My alternator and other charging sources charges ONLY the house bank. W
I use a Blue Seas switch which has separate switches for the start and house bank. The house bank switch is a 100 amp breaker and the start batt switch is a 250 amp IIRC rotary switch. There is a combine switch on the panel as well which essentially lets me use the house bank to start with, though I never have.
The problem with any parallel battery wiring
is that a bad battery can suck down the good one. An one two both does parallel the two banks.
And how does the charging source determine which battery in a parallel set up is weaker. If one bank is quite charged it will mask the discharge of the weaker battery.
My start batt does one thing - crank the starter for a few seconds. I have removed all associated power loads such as engine instrumentation and the engine ventilation from the start bank. The blower was a vary large continual drain when motoring, larger than the echo could match so the start would slowly discharge from motoring. So now start batt has a very limited purpose - crank the starter.
This system has worked well.
One thing that is important about multiple charging sources. You need to have a charge controller which can manage them or use them separately, ie turning off the least powerful charging source. So if you have a small solar
cell and it's a bright day and you start your engine, and the alternator is looking to replenish the system, if it sees an elevated voltage from the solar
trickle it will "think" the batts are quite topped up while they might be fairly discharged, but on trickle charge. Alternator would politely defer! Solution is to kill the solar charge when you use the poweful alternator.