If you need batteries you'll want to change out both.
You also need to check this: if they're wired pos-pos and neg-neg that's likely 2 12V batteries wired in Parallel. If yours are wired pos-neg from battery A to Battery B that is 2 6V batteries wired in Series.
You must know for sure so you don't accidently double up to 24V.
Gary
GVP
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