A.T. Robertson Commentary Mark 6:7

"And he calleth unto him the twelve, and began to send them forth by two and two; and he gave them authority over the unclean spirits;" — Mark 6:7 (ASV)

By two and two (δυο δυο). This repetition of the numeral instead of the use of ανα δυο or κατα δυο is usually called a Hebraism. The Hebrew does have this idiom, but it appears in Aeschylus and Sophocles, in the vernacular Koine (Oxyrhynchus Papyri No. 121), in Byzantine Greek, and in modern Greek (Deissmann, Light from the Ancient East, pp. 122f.). Mark preserves the vernacular Koine better than the other Gospels and this detail suits his vivid style. The six pairs of apostles could thus cover Galilee in six different directions. Mark notes that he "began to send them forth" (ηρξατο αυτους αποστελλειν). Aorist tense and present infinitive. This may refer simply to this particular occasion in Mark's picturesque way. But the imperfect tense εδιδου means he kept on giving them all through the tour, a continuous power (authority) over unclean spirits singled out by Mark as representing "all manner of diseases and all manner of sickness" (Matthew 10:1), "to cure diseases" (ιασθα, Luke 9:1), healing power. They were to preach and to heal (Luke 9:1; Matthew 10:7). Mark does not mention preaching as a definite part of the commission to the twelve on this their first preaching tour, but he does state that they did preach (6:12). They were to be missioners or missionaries (αποστελλειν) in harmony with their office (αποστολο).