Gene,
Typically a Group 31 AGM is about 105 amp/hours. Two in parallel will be 210 amp/hours. A Group 24 starting battery will be about 65 amp/hours at minmum. So the collective amp/hours will be 275+. For proper desulphation after deep cycles I recommend a 30 amp charger. A 20 amp charger is lite. If you have an AGM starting battery also, so all battery chemistries are the same, then we use the Intelli-Power 30 amp.

David