The angel ordered Mary and Joseph to name their son the perfect name of Jesus because it means "Yahweh saves", which is exactly Jesus's mission. Jesus's mission is to save us from our sins.
I think that the angel told them independently because it would show just how important it was that this be carried out. The angel had their undivided attention, and it was even more personal this way. I think about a professor giving instructions to a class of 10, 30, or even 100 students. How many people walk away actually understanding the lesson/instructions the professor, in detail, had given them? There are distractions all around, and maybe even fear of asking a "stupid" question in front of others for clarity. Then I think about that same professor taking time out to go over the instructions of an assignment with each student, with undivided attention, and room for questions if needed. That student knows exactly what they are expected to do. At first, I did not fully understand the importance of the angel talking to Mary and Joseph, each on their own, until I looked at it this way. How powerful even the smallest act or detail is!