Portable battery Charger

I would suggest using a "Smart" type charger (I used a Deltran Battery Tender 2/20/70 amp model (20 was for battery charging then it dropped to tender) you do need to monitor it as it will not return to "Charge" automatically.. 70 is for jump starting and did not work well for me).

The most likely "worst case".. one of the chargers may refuse to go into "Battery charge" because it senses the other charger and thinks "Batteries full"

Other worst case.. already covered above.