Are you talking about charging/maintaining your gensets batteries while unit is offline and not running? If so, you can use a 24V charger/maintainer for the two 12V batteries connected in series (24V configuration). Or, you can use two 12V charger/maintainers connected to each battery. I prefer to use two 12V charger/maintainers so that each battery is measured and charged independently so it gets exactly what it needs. If one battery were older or had a marginal cell it may required additional charging whereas the other battery might not. I think of a 24V charger as averaging the required charge between both batteries when connected in series.
Also, when using two 12V charger/maintainers it is not required to disconnect the battery leads (they can remain connected in series). The battery charger/maintainer only sees what is connected between its two +/- charging leads.
Obviously, for any genset model that utilizes a single 24V battery you must then use a 24V charger.