There are commercial dual chargers such as the adverc and silverline systems, they are very expensive.
If your charger produces enough voltage, like 14.5v or more you can use a double diode, you can buy these from chandlers too and they are much cheaper.
Using a double diode with a 'dumb' type charger isnt so bad, the voltage drop across the diode doesnt affect the battery charge that much, allthough the charger will charge the lowest battery first, and if one is dead then both dont get charged.
Using a double diode with a alternator on the other hand isnt good as the voltage drop across the diode does have an effect, the batteries instead of being charged to 13.8v are charged to 13.8v - 0.6v (diode drop) = 13.2v which cam have an effect on bettery performance.