Albert Barnes on Mark 6:7

By two and two. In order that they might support and encourage each other in their work. Amidst the trials and opposition which they would meet with, mutual counsel and aid would greatly lighten their burdens, and alleviate their calamities. Mutual counsel might also contribute to their success, and lead to united plans to advance the kingdom of the Redeemer. Jesus here, as in all the work of religion, consulted at the same time the happiness and usefulness of his disciples. Nor are they ever separated. Whatever contributes to the usefulness of the people, produces also their happiness; or, in other words, the secret of being happy, is to be useful.

(b) "And he called unto him" Mt 10:1, Mk 3:13, Lk 9:1, 10:3