The Meaning of Numbers: The Number 2
The number 2 conveys the meaning of a union, division or the verification of facts by witnesses. A man and woman, though two in number, are made one in marriage (Genesis 2:23 - 24). There is also the union between Christ and the church (see 1Corinthians 12).
The testimony of God is divided into Old and New Testaments. His agreements with mankind are divided into Old and New Covenants. And, though 2 in number, God the Father and God the Son (Jesus Christ) compose a single Godhead.
The first man, Adam, sinned and brought death and destruction into the world. Jesus, however, as the second (or last) Adam brings the hope of the resurrection and eternal life (1Corinthians 15:21 - 22, 45 - 49).
Those who ultimately refuse to repent and obey God will be put to death forever by being thrown into the lake of fire, which is called the second death (Revelation 21:8). This act will eternally divide those who are righteous from those who practiced evil.
At the minimum, the testimony of at least 2 people were needed in the Old Testament to convict someone of a crime or sin. The validity of this teaching of 2 people needed was confirmed by the Apostle Paul (1Timothy 5:19, Titus 3:10).
In the End Time, two witnesses will appear on the world scene to testify and uphold the truth of God against the Beast and the False Prophet (see Revelation 11). Jesus sent the disciples out in groups of 2 so that they could not only testify about his teachings and miracles, but also be witnesses of those who accepted or rejected the gospel (Mark 6:7 - 13).
Two can also convey comparison and contrast. The Old Covenant was based on physical blessings for obedience while the New is based on those that are spiritual.
In the Old Testament physical circumcision was required in order to be considered part of God's people, while the New requires circumcision of the heart. God formerly required obedience only to the "letter" of his laws but now calls upon man to obey their spiritual intent as well.
Appearances of the number two
The word 'God' appears in all books except 2, which are the Song of Solomon and Esther.
The shortest verse in the Bible, which is "Jesus wept" found in John 11:35, contains just two words. Of the ten shortest books, eighth ranked Haggai has only two chapters.
During Creation week God made two great lights, one to rule the day (sun) and a lesser one to rule the night (moon - Genesis 1:16).
How is the number 2 significant?
Where there is a second epistle in the New Testament it is contrasted to the first through, in part, its discussion of an enemy. Hence, one subject of the book of 2Corinthians is the devil (2:11, 11:14, 12:7).
In 2Thessalonians we have a number of titles and references given for Satan. They are the 'man of sin' and 'son of perdition' (2Thessalonians 2:3), the 'mystery of iniquity' (or lawlessness in 2:7) and that 'wicked one' (2:8). In 2Peter we have an End Time rejection of God and the truth foretold (2Peter 3:3 - 7). John warns us in 2John about deceivers whom he also refers to as antichrists (1John 1:7).
Additional info on Biblical Meaning of 2
The second of the three great divisions of the Old Testament is referred to as 'the Prophets' (Luke 24:44).