There are a number of differences between the synoptic Gospels, which suggest that the accounts, while similar, are distinct.
Matthew begins his account relating the genealogy of Joseph - Jesus’ adopted earthly father. He then connects the events surrounding Jesus’ Birth to Old Testament prophecies, demonstrating how they have been fulfilled in the events he’s just described.
Mark, on the other hand, begins with the ministry of John the Baptist - an event taking place when Jesus was in his early thirties.
Different beginnings imply separate yet distinct authors.