Why are two blues used in a split-primary palette?

Correct answer: A greenish blue is useful for greens and a reddish blue for violets

The governing concept is colour bias in a split-primary mixing system. A greenish or cyan-leaning blue combines with yellow with less red influence, producing a cleaner green. A reddish or violet-leaning blue combines with red with less yellow influence, producing a cleaner violet. The pair does not make yellow, create white by itself, or necessarily have identical temperature.