Gregory the Great — as excerpted in the Catena Aurea on Mark 6:6-13

“(Hom. in Evan. 17) Further, the Lord sent the disciples to preach, two and two, because there are two precepts of charity, namely, the love of God, and of our neighbour; and charity cannot be between less than two; by this therefore He implies to us, that he who has not charity towards his neighbour, ought in no way to take upon himself the office of preaching. There follows, And he commanded them, that they should take nothing for their journey, save a staff only; no scrip, no bread, no money in their purse: but be shod with sandals; and not put on two coats.”