If the Blue Sea ACR gizmo is as smart as the Yandina/West combiner is, that scenario could not happen!
With a big enough alternator, say a Balmar 100 and a multi-stage regulator, you'd be hard pressed to see it ever drop bellow the 13.3 volts it takes to initiate the relay closing in the bulk phase.
Check the manual. If the house bank is so far depleted that it could suck the alternator and battery voltage below 13.6(?) volts--the combiner will open up and isolate the starting battery from the house bank again.
Yes the Yandina 50 & 150 do that but it begins the disconnect at 13.3 volts as opposed to 13.6 and then has a 30 second delay which I forgot about. My bad on the delay!
IIRC there's about a 90-second delay in the system so it doesn't chatter, but when the system voltage drops, for any reason, the Yandina combiner cuts out the second battery until the primary one has been brought back up to nominal voltage again.
Actually 30 seconds and yes I forgot about that, that's why I see my combiner cut out at 13 volts not 13.3..
The only real fault I've heard is that, after all, a relay can fail. And I agree, they can fail, but they're pretty damn reliable when they are sourced from prime sources to begin with. (As both the Yandina and BS probably are.)
Never had one fail!
P.S. RXBOT is right about the cut out voltage being too low for some float stages! I see my combiner cut out at 13 volts but that is after a 30 second delay from the 13.3 cut off hence my thinking it was 13 volts.
Sorry for my bad info on this! If you have an adjustable float voltage Yandina recommends upping it to 13.4-13.6 instead of the usual 13.2-13.4 for wet cells. My float drops to 13.2 so I'm most likely not float charging my start battery, will need to check it.
Darn 30 second delay screwed me all up...!