Gregory the Great — as excerpted in the Catena Aurea on Luke 10:1-2

“(Hom. 17. in Ev.) He sends the disciples to preach two and two, because there are two commands of charity, the love of God, and love of our neighbour; (and charity cannot exist without at least two;) thereby silently suggesting to us, that he who has not love to another, ought not to undertake the office of preaching.”