Place vegetables in a slow cooker and put chicken on top breast down. Fill with water halfway up the chicken. Cook on low for 8 hours.
Remove the chicken from slow cooker. Take the chicken apart, reserve meat and skin for another dish. Leave 1st broth and bones to cool.
Roast the beef bones for 20 minutes in an oven preheated at 400F. Spoon marrow out.
Add chicken and beef bones, 1st broth, vegetables and the apple cider vinegar to the slow cooker and add water to cover the bones by about an inch.
Let the mixture sit for 30-60 minutes without turning on the heat. Leaving the mixture sit while cool will permit the vinegar to extract the minerals from the bones.
Simmer in the slow cooker on low for 24 to 36 hours. Top with water if too much evaporates.
Strain the broth and salt to taste (I only used 3/4 tsp). Let it cool in the fridge and skim the fat off the top once hardened. Your broth is ready.
Notes
The broth is ready when the chicken bones crumbles when pinched between your fingers. Beef bones can be used for 3 rounds of broth making.