I think that whatever way works for you, and you're comfortable with, is the way you should do it. You guys had me so scared about "mitered corners" that I practiced on practice material several times before I did it on a real quilt. And, I might add, did them perfectly several times before I did them on a real quilt. Was still quaking when I did mitered corners on a real quilt. They still came out perfectly. That's not saying they're easy, but sometimes you scare us Newbies (to quilting) too much!
Putting two borders together before putting them on the quilt -- don't see the point -- not in my comfort zone. On the other hand, there is something to be said about getting outside one's comfort zone! Hmmmm. . .
Cheers!