Question: "Why are there two demon-possessed men in the Gerasene tombs in Matthew, but only one in Mark and Luke?"
Answer: The three passages that describe the incident with the demoniacs in the country of the Gerasenes, also called Gadarenes, are Matthew 8:28-34, Mark 5:1-20, and Luke 8:26-39. The Matthew account mentions two demon-possessed men, while Mark and Luke only mention one. Is there a discrepancy in these accounts, and do the Gospel writers contradict one another?
The first thing to determine is whether the three writers are describing the same event. The timing of the event in all three accounts—immediately following the calming of the storm on the sea of Galilee—as well as other similarities (living in the tombs, the ferocity of the demoniac, the conversation with the demons, the driving of them into the pigs, the drowning of the herd, and the response of those who witnessed the scene) all give credence to Matthew, Mark, and Luke all describing the same event. The question remains, then, whether there was one demoniac or two.
Matthew tells us there were two demoniacs, while Mark and Luke only mention one of the two. It is unclear why they chose to mention only one, but that does not negate the possibility of a second demoniac being present. Mark and Luke do not say there was “only one” demon-possessed man. They simply state that one of the two met Jesus and spoke to Him. For whatever reason, Matthew simply gives us more information than Mark and Luke.
In any case, no contradiction exists. A contradiction occurs only if one statement makes the other impossible and there is absolutely no way for them to be reconciled. For example, let's say we put two apples on a table. Statement 1: There are two apples on the table. Statement 2: There is only one apple on the table. These two statements contradict each other. Now read these two statements: Statement 1: There are two apples on the table. Statement 2: There is an apple on the table. These two statements do not contradict each other. In the same way, the biblical accounts do not represent a contradiction. All three accounts describe demon possession and the power that Jesus has over the spirit world. All three tell us that He made a point to cross the sea to save someone from the demons. All three affirm that there was at least one man who was plagued by demons. The fact that the three accounts differ in some minor details only proves that they were written by three different authors, each of whom chose to focus on a different aspect of the account.