Get at least 5A per bank, preferably 10A. The more quickly you get the battery charged (up to a point) the longer the battery will last. Most onboard chargers are actually 3 individual chargers that don't care what the other banks are doing. Some now have the ability to charge whatever battery needs it.

Onboards do not operate as trickle chargers when the battery is charged, they are float-mode chargers. Float-mode maintains a low charhing voltage to reduce gassing while monitoring the battery current draw, while trickle chargers are typically around 2A fixed current which can boil a battery dry.

A 2A charger will struggle to charge two batteries in parallel. As they age, losses during charging increase until 1A per battery won't do much but make the battery warm. You should charge at 10A if that's available, or even 15A if your charger can do it. Don't trust a charger that says automatic or manual. These things will destroy more batteries than anything else. Use a charger that says "Smart Charger" or "3 or 4 Stage Charger" to get full charges and longer battery life.