As a noun, pair can be followed by a singular or a plural verb, but the singular is always used when pair emphasizes unity or oneness: "This pair of shoes is black." A plural verb may be used when the members of a pair are treated as individuals: "The pair are running rapidly now." After any numeral other than one, say pairs, not pair: "Sue bought three pairs of stockings." Do not say "a pair of twins" unless you are referring to four people.
Dictionary of problem words and expressions. Harry Shaw. 1975.