ANSWER: To answer your question, I find just one reason for Christ sending out his disciples two by two as a sort of 'training exercise' for preaching the gospel. This reason is that two people provide a valid witness instead of just one person (Matthew 18:20).
7 He called the twelve disciples together and sent them out two by two. He gave them authority over the evil spirits . . . (Mark 6:7)
Note that in Mark 6:7 not only was Christ with them in Spirit, He gave them His Spiritual power to do His work of spreading the gospel!
The disciples were called not only to be Jesus' companions but also to be his witnesses to all of His teachings, miracles, etc. They were called and chosen to give a first-person testimony about Jesus' whole ministry.
Jesus' disciples were sent out by twos so that together they could be witnesses as to whether those who heard them accepted or rejected the gospel. If they went out alone one person's word without a witness would not be acceptable in the things of God (see 1Corinthians 14:29, 2Corinthians 13:1). The word of one person is not enough to establish the validity of anything!
The apostle Paul told his best friend Timothy to not accept an accusation against an older person (whether in or outside the church) unless there were at least two or three witnesses (1Timothy 5:19). And, Paul makes it plain in the book of Hebrews that it took the words of two or three people to condemn someone to die for rejecting God's laws (Hebrews 10:28).
No person could be convicted of a crime in the Old Testament by the testimony of just one witness. Two or three witnesses were needed to decide whether a person committed a crime or not. A person's honest testimony was taken so seriously that if THEY were found to be a false witness (they lied about what they heard and saw) THEY would receive the same punishment as the one they testified against, even if the punishment was death!
Jesus also instructed the disciples he sent out to give the people of a village or town a sign if they refused to hear what they had to say.
11 If you come to a town (this instruction was not for an individual household that might reject the gospel) where people do not welcome you or will not listen to you (they refuse to hear the gospel), leave it and shake the dust off your feet. That will be a warning to them! (Mark 6:11).
The phrase "shake the dust off your feet" is similar in meaning to what the Eternal said to Israel in Deuteronomy 30:19. The shaking of dust from their (the disciples') feet symbolized that even the earth (the dust on their feet) was a witness against those who would not hear the people God sent to preach his gospel.