When he arrived at the other side in the region of the Gadarenes, two demon-possessed men coming from the tombs met him. They were so violent that no one could pass that way. Matthew 8:28

When Jesus stepped ashore, he was met by a demon-possessed man from the town. For a long time this man had not worn clothes or lived in a house, but had lived in the tombs. Luke 8:27

Matthew 8 does not contradict Luke 8 by mentioning two demoniacs. If one story said it was a demoniac and the other said it was a cow, that would be a contradiction. A contradiction shows something that is contrary, opposite or opposed to the other statement.

If an eyewitness described a bank robber and another described two bank robbers, we would not call that a contradiction. One eyewitness simply reported more information than the other. Their perspective or point of view differed. The first could have reported a more detailed description on the one bank robber, while the other focused on two of them.

Luke 8 focuses specifically on one of the two demoniacs and what happened to him when Jesus and the disciples showed up in his area. The story is really about the one demoniac and the dramatic change he experienced. Matthew focuses more on the incident and mentions both demoniacs that were present. Luke is a personal account of what happened in the life of one demoniac, leaving the other demoniac in the background. Matthew is more news reporting, talking about the event. Luke is a character study.

The same contradiction theory is used when the Gospels described the angels in the empty tomb. Matthew describes one angel. Luke says there were two. Again, these are not contradictions and the gospels can be trusted.