Indeed, the argument that they were different persons is a good one. Mark was the earliest written account, and yet he was not one of the 12 Apostles. Most reputable scholars estimate that Mark's Gospel was written between AD 65 and AD 75. So several decades after Jesus' death. So this is stretching time for the others to have sight of Mark's Gospel, or a possible common source (the hypothesised "Q"). But not necessarily beyond the realms of the possible. Like I say, there's no evidence either way. I don't personally happen to believe they are the same Matthew and John, but only on balance.