Seems a bit daft not having a couple of kg's of nicads as a backup for the standby AH.
What are the main bat's, nicads or lead acid?
Do they stagger the alternator maint or do you usually replace/overhaul both at the same time?
Personally with that set up I would be wanting one alt that was half lifed and one new both from different batches.
Unfortunately there isn't a easy solution to the fact that the alternators both have a common earth which means if one goes if can back spike the earth and take the other one out.