In these times naming carried a lot more power than it would nowadays, that's why several name changes happened I guess - like Jacob became Israel, Saul became Paul to demonstrate transformed natures...
In this case the name by definition determines the exact and deliberate purpose of the individual... the fact that the prophesies had promised a messiah to save the people from their sins and an angel speaks to both Mary and Joseph telling them to call the Holy Child Jesus is absolute proof of the nature and purpose of the baby.
To tell them both independently may be for proof, for emphasis but also, although they were called for a joint purpose, to nurture, protect and bring to maturity the expected Son of God, they were also called as individuals, each role was vital to the overall outcome... it was equally important that Joseph played his part as willingly as Mary... to be given instructions by an angel would certainly ensure compliance, surrender, service no matter what.