750 -800 ma wall wart just like Westend above describes is perfect for a 100-amp hour battery and not even in the ballpark with two 100-amp batteries. This size wall wart is common as can be and you can get one cheap at allelectronics.com or MPJA.com
These things work continuously for years and years and don't break. For flooded batteries only. A 400-ma may be enough for an AGM battery but thorough testing of final voltage float is essential. For cold weather hibernation a one amp 15 volt model may fit the bill. But for max life a top charge every 2-3 months is recommended for a flooded battery.