I'm afraid it is not possible entirely to harmonise the different accounts.
Accepting the priority of Mark, which says there was one young man in the tomb when the women arrived, it is possible to say that Mark's informant was simply unaware of the second man reported in Luke's Gospel. Going from Luke to John, the only change is that the two men in shining garments explicitly become two angels whom Mary Magdalene saw inside the sepulchre. So far, so good: we can say that there were two of them and that the authors of Luke and John became aware that Mark got it wrong.
Matthew's Gospel, working from Mark's Gospel independently of Luke, stays with the one person, but not inside the tomb as in the other gospels. Instead, the women watch an angel descend from heaven and roll the stone from the door. This miracle account can not be harmonised, if only because the angel did not arrive until the women were present.
The four gospel stories of the empty tomb are so different as to prompt Archbishop Carnley, former Anglican primate of Australia, to say in The Structure of Resurrection Belief:
The presence of discrepancies might be a sign of historicity if we had four clearly independent but slightly different versions of the story, if only for the reason that four witnesses are better than one. But, of course, it is now impossible to argue that what we have in the four gospel accounts of the empty tomb are four contemporaneous but independent accounts of the one event. Modern redactional studies of the traditions account for the discrepancies as literary developments at the hand of later redactors of what was originally one report of the empty tomb...
There is no suggestion that the tomb was discovered by different witnesses on four different occasions, so it is in fact impossible to argue that the discrepancies were introduced by different witnesses of the one event; rather, they can be explained as four different redactions for apologetic and kerygmatic reasons of a single story originating from one source."
Even for those who do not accept Markan priority, Archbishop Carnley is saying that no less than three of the authors redacted an original story for apologetic and kerygmatic reasons. We could say that because Mark, the first New Testament gospel, reported only one young man, then that is the correct number, or we can say we simply do not know how many persons there were.