Since the maintenance chargers require 120vac to work, why don't you just plug the mh into that and let the mh's on-board charger keep the batteries charged?
However, to answer your questions:
1. Yes, a single charger will keep both batteries charged..no need to separate.
2. Yes, the same charger will work on multiple batteries.
However, keep in mind that maintenance chargers are typically very low amperage and if the batteries are not fully charged when you connect the maintenance charger, the charger may not have sufficient amperage to bring the batteries back up to a full charge.
If you are connected to shore power and your on-board charger keeps your house batteries charged but not your chassis batteries, invest in a little device called a "Trik-l-start". It does not require 120vac to work and will enable your on-board charger to keep ALL your batteries fully charged.
Hope this helps