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The two cells when in series make s 12 volt source. Thus assume you have 12 volts 12 amps battery. The charger needs to have an potput of 13.5 or 13.8 volts to get it charged and the battery voltage may rise up to that level white bieng charged. As the capacity 12 Ah, the safe limit is 10% of it ie, 1.2 amps for charging.
You'll find different numbers depending on the battery type and charging method. Another standard specification, that can be e.g. found with Panasonic batteries is 0.15 C charge current for trickle charge and 0.4 C in cycle use.