Charging Capacity and Amperage of Charging Unit

Nor damage the battery, as long as the same technology is used.

It's no different from filling a 5 gallon bucket with water at a constant rate. To fill the bucket takes X minutes.

To fill two 5 gallon buckets will take 2X minutes.

If you're charging 2 batteries in series, they should have the same Amp Hour ratings and be in the same state of discharge.

A lot depends on how smart your charger is and what charging strategy is employed....

..."If you charge your battery at a 10-amp rate and you need to replace 36 amp-hours in the battery. To determine how long (in hours) you will need to charge your battery, divided 36 by 10 = 3.6 or 3.6 hours to recharge your battery.

A 12 volt / 10 amp 3-stage automatic charger is the best charger for most automotive batteries. Unfortunately most chargers do not output the entire 10 amps during the charge cycle, because it automatically limits the voltage and the amperage during the charge cycle. You may actually only see about 1/2 the output over the time period that you are actually charging the battery. For that reason, it can easily take 12 hours or more to fully recharge the battery. Even after 12 hours, because of the reduced voltage, the battery may require more charging to get it 100% charged."...

http://batteriesbyfisher.com/determining-charge-time

As long as you stay within the same battery chemistry the charge detecting circuitry, if any, should work fine. It will just take longer to charge.