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It is best to separate the 2 different batteries, the charge profiles are not compatible. Keep in mind that the Lithium (assuming you mean Lithium iron phosphate such as Battleborns) will accept a much larger charge rate than the lead acids. Current thinking is to use a DC to DC charger. Victron and Sterling make really nice ones. This effectively isolates the two different battery types and limits the amps going to your LIthiums effectively protecting your alternator and wiring. If you'll be doing any boondocking you'll love the Lithiums!

Richard beat me to the punch. B2B is the same as DC to DC charger.