There are many on-board chargers/maintainers that are designed for dual batteries. Promariner being one of the best. I'm not aware of a "portable" dual charger. Of course you could buy two small maintainers fairly cheap. Recommend you go with the newer "smart" technology chargers which are better for battery life. If you need to recharge a dead or low battery, then a full size charger of 10-20 amps is needed. Guest, Noco, Schumaker, etc. There are several on the market.