Looking for authentic Vietnamese Bun bo hue recipe, please, and question about using bones

I'm looking for an authentic Bún Bò Huế recipe, please. I keep finding recipes that use chicken broth (it translates to spicy beef soup...).

Also, many of the pho and other Vietnamese soup recipes call for at least two different types of bones. Usually both oxtails and beef leg bones.

I'm confused about why. They seem like pretty similar cuts with marrow. So what's the point of using both?

bon appetit