That's not how it works. You can parallel 12V lead acid batteries of the same state of charge. The main concern is not overcharging the batteries. After the bulk charge (around 14.5V), the voltage should be reduced to a float charge level (around 13.7V). Many battery chargers can do one or the other but don't switch to float when the battery is charged. Look for smart chargers which give their voltage specification for each mode.

edit: AGM gel and flooded batteries may have slightly different voltages. It is best to use a charger designed for the chemistry of the battery.