“He summoned the Twelve and began to send them out two by two.” (Mark 6:7)

Jesus instructed them to take nothing for the journey but a walking stick.

Consequently they went off and preached repentance. They drove out many demons and  anointed with oil many who were sick and cured them.

Let us look at the moment after Jesus’ Ascension: 

'Nw Peter and John were going up to the temple area for the three o'clock hour of prayer.

And a man crippled from birth was carried and placed at the gate of the temple called "the Beautiful Gate" every day to beg for alms from the people who entered the temple.
When he saw Peter and John about to go into the temple, he asked for alms.

But Peter looked intently at him, as did John, and said, "Look at us."

He paid attention to them, expecting to receive something from them.

Peter said, "I have neither silver nor gold, but what I do have I give you: in the name of Jesus Christ the Nazorean, (rise and) walk."

Then Peter took him by the right hand and raised him up, and immediately his feet and ankles grew strong.' (Acts 3:1-7)

Note that Peter and John preached and acted together.

Why did Jesus ask His disciples to go out and preach in pairs?

It was simply because when Jesus wanted them to practice mutual love while spreading the message of the gospel, they had to first practice it themselves or else it would be empty language.

Unsurprisingly, Paul did not invite Mark to join during the second missionary journey. Paul was discontented with the latter leaving without a reason during the first travel. (During the latter phase, they became friends again in Rome; Mark even became the personal secretary of Paul.)

In addition, if we pray with our hearts and mind together, Jesus will respond to our prayers and be present in the midst of the community that has mutual love.

“Amen, I say to you, if two of you agree on earth about anything for which they are to pray, it shall be granted to them by my heavenly Father. For where two or three are gathered together in my name, there am I in the midst of them."  (Matthew 18:19-20)

Is there anything more important than having Jesus present in the midst of us?