This is something that is rarely seen in Hanoi or Saigon (HCMC), and honestly something I would normally try to steer you clear of. BUT…. In Da Nang, there are some Pho shops who combine Beef & Chicken into one single bowl of soup. I believe the stock is made from both as well. You never cease to surprise me Da Nang. PHO TIP: In other regions of Vietnam, I highly suggest that you not only stay away from shops that combing beef and chicken pho, but also shops that serve both under the same roof.