I think you should harvest the Juneberries and mix them with the
unripe Buffaloberries while you are waiting to can them. That would be
the equivalent of using lemon or lime juice as an antioxidant to
retard ripening (crystal citric acid works well too). Usually in
nature there is some hidden mysterious synergy between adjacent
plants. Separate them physically and spoilage is hastened.

In any case, we always used to mix fast ripening fruit with sour
underripe fruit just to give us more time to process it.

-- Gnarlie