Theophylact of Ohrid

Furthermore, not only did He Himself teach, but He also sends His disciples, and moreover in pairs of two, so that they might be bolder. For if He had sent them one by one, one alone could not have acted so boldly, and if He had sent more than two together, the number of apostles would not have sufficed for all the villages. So He sends them two by two: "Two are better than one," says Ecclesiastes (Eccl. 4:9).

Commentary on Mark ↗

Gregory the Dialogist

Hence also miracles were added to the holy preachers, so that the power displayed might give faith to their words, and that those who preached new things might do new things. When the world was flourishing, when the human race was increasing, when the flesh was subsisting long in this life, when the abundance of things was overflowing, who upon hearing would believe that there was another life? Who would prefer invisible things to visible things? …

Forty Gospel Homilies, Homily 4 ↗

Bede

Now our kind and merciful Lord and Master did not grudge His servants and their disciples His own virtues, and as He Him self had healed every sickness and every infirmity, so also He gave the same power to His disciples. Wherefore it goes on: And he called unto him the twelve, and began to send them forth by two and two; and gave them power over unclean spirits. Great is the difference between giving and receiving. …

Catena Aurea by Aquinas ↗