Good point about one battery taking out the other. My wife's Alfa Romeo
164 used to use a huge battery (for tractors), but it would be way
overkill in terms of replacement cost versus time it may potentially be
used. I hope to get a small generator for camping, so I'm thinking I
can use that for emergencies past 8 hours. I'm more concerned about
the autmatic cutover when the AC sump goes out/off.

Actually, there is a place where it should drain out. Even though my
basement is 6 feet underground, there is a 15 degree slope going down
the road and my house is above the sewer level, so it would drain out
under ideal situations assuming water only comes through the sump area
and not through other parts of my foundation. I have a floor drain 4
feet from the sump pit, but I'm worried about major flooding that might
cause the sewers to backup into the house. I have an aftermarket
device in the drain to prevent backflow, but a congruence of calamaties
(power out, big rain storm, overflowing storm drains etc.) could reak
havoc and the resulting "wife storm" damage could be much worse, so
hence the battery backup.