I dont think they have always been like that tho. I know from v15 on has. We are on v18 now.
A battery is considered "charging" above 12.7 volts. As long as one (not both) is above this threahhold they should remain connected.... i think!
Once one drops to 12.7, it pulls opposite direction until both are at 12.7 for [x] seconds. Then the isolator opens and stops the bridge. Thus isolating the 2 batteries from each other.
This is what TrueAm has said about it anyways.