Which books in the bible are written in pairs?

There are a number of these in both the Old and New Testament. The Old Testament books of 1 & 2 Samuel, 1 & 2 Kings, and 1 & 2 Chronicles belong in pairs and were probably originally written as one whole book.

Luke's two books of the Gospel of Luke and the Acts of the Apostles were stated as being written separately although both addressed to the same person, possibly someone who either requested the work or who sponsored its production and distribution, namely one Theophilus.

Other New Testament works which could be paired are the two letters of Peter, 1 & 2 Peter, as well as 1 & 2 Timothy and 1 & 2 Corinthians, written by Paul and paired because written to the same person or group.