Fill a large stockpot with 3 cups of water and bring to a boil.
Place the crabs in the water and steam for 15 minutes or until their shells are orange-red and their bellies turn white.
Save the crab stock. Remove the meat from the body and claws, and save the juice from the body.
In a separate stockpot, fill 10 cups of water and cook the pork neck bones for 2 hours on medium heat.
For the soup, pour the strained the pork stock, 2 cups of crab stock, and juice from the crabs into a separate stockpot. Add fish sauce. Let the soup simmer on low heat for 30 minutes and remove any scum at the surface.
Season with salt and sugar to taste. Add 1/2 a cup of water at a time if the soup becomes too salty for your taste.
Boil the tapioca noodles until semi-clear. Strain and leave in cold water so they do not expand.
When ready to serve, add half a cup of noodles to a serving bowl, top with crab meat and shrimps and then add soup.
Garnish with green onions, cilantro, pepper, and Bird's eye chile pepper.