That's assuming Mark had Matthew's gospel before him. Of course they could have been written independently of each other.
Are you suggesting some independent means of oral tradition, perhaps? This would suggest some very good memories, because of the identical content of so much of the two gospels. Or some other written source? I'm assuming you don't think the evangelist Matthew was actually the
disciple Matthew? Occam's Razor, scraped every morning, is recommended. Twice a day if there's five o'clock shadow.
In fact, as I'm sure you know, the only substantial tradition we have about the gospels comes via Papias about Mark's, and that's none too certain:
The Elder used to say: Mark, in his capacity as Peter’s interpreter, wrote down accurately as many things as he recalled from memory—though not in an ordered form—of the things either said or done by the Lord. For he neither heard the Lord nor accompanied him, but later, as I said, Peter, who used to give his teachings in the form of chreiai, but had no intention of providing an ordered arrangement of the logia of the Lord. Consequently Mark did nothing wrong when he wrote down some individual items just as he related them from memory. For he made it his one concern not to omit anything he had heard or to falsify anything.
The information regarding Matthew is much more nebulous, with some reference to a Hebrew version. (As for John, we know that it was the favourite of the gnostic Valentinus - which I wouldn't have thought you'd be too pleased about). Another alternative is divine inspiration for one or the other (can't be both, because of the differences and indeed contradictions, unless God really is confused)