Thank you for the question on Reuben’s son, “Did Reuben only had two sons in Genesis 42?”
Genesis 46 states that Reuben had four sons who went down into Egypt. “These are the names of the sons of Israel (Jacob and his descendants) who went to Egypt: Reuben the firstborn of Jacob. 9 The sons of Reuben: Hanok, Pallu, Hezron and Karmi” (NIV).
This poses a question of why did Reuben say to his father, Jacob that he may kill both of his son if he does not bring Benjamin back? As I look at the context, Jacob is bemoaning the loss of Joseph and Simeon and the dread of losing Benjamin would be too much to bear. Jacob’s feeling is somewhat unwarranted for Reuben would have share that Simeon is imprisoned until their youngest son joins them on this trip to Egypt. Reuben is attempting to comfort and to assure that Reuben will do everything in his power to bring back Simeon and Benjamin to him. If he fails, he is willing to sacrifice two of his sons. That was to show to Jacob that the task before him is a serious one. He would be willing to forfeit two of his sons but one knows that Jacob would not kill his own grandchildren (Genesis 42:35–38).
SUMMARY: Reuben had four sons who went into Egypt without any indication if he had other sons in Egypt through his wife or other wives that he may have married.
For more perspectives:
https://www.quora.com/Did-Reuben-only-have-two-sons-in-Genesis-42