[RMcNeal]

As Carl said, when wired in parallel, two 12v batteries will give you twice as many amp hours compared to only one. Since you should not draw lead acid or AGM batteries below 50%, the usable amp hours would be that of one battery. For example, one 12v-100AH battery gives you 50AH that are usable. Two 12v-100AH batteries will give you 200AH total, but only 100AH that are usable. That's without risking damaging the batteries by drawing down below 50%. The charging profile for flooded lead acid and AGM batteries are virtually the same, since AGM batteries are still lead acid but use and "Absorbed Glass Mat" instead of flooded electrolyte which makes them "so called" maintenance free. Any working charger you have for lead acid batteries will work fine for the AGM batteries as well.