You'll need a charger with more amperage capacity than the average trickle charger- - - - -maybe one with a max output of 10-15 amps. Most real trickle chargers have a capacity of 5 amps or less. The battery with the lowest specific gravity will hog most of the charge current until the one with the lower state of charge comes up to the level of the higher-charged one.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!