Open the chowder clams, making sure to catch all the clam juice. Mince the clams and reserve them. Measure the juice; it should come to about 2 cups. If it doesn't, make up the difference with bottled clam juice. Then add 1/2 cup water, making 2 1/2 cups liquid in all.
Pour the olive oil into a wide heavy pot over medium heat. Add the minced garlic and saute for 4 to 5 minutes or until the garlic just starts to turn gold. Add a little clam juice to loosen the garlic then add the rest of the 2 1/2 cups. Raise heat.
Salt the boiling water lightly and add the capellini, stirring well. Cook until just al dente, about 7 to 8 minutes.
Simmer the clam juice in its pot for two minutes. Raise heat and add the Manilla clams. Cover. Within two minutes or so, their shells should swing open; remove the Manilla clams from the pot and place them in a bowl. Turn heat to low under clam juice in pot and add the reserved, raw, minced clams. Cook very gently for 1/2 minute.
The pasta should be done at this point. Spill the pasta into a colander and place the cooking pot over a low flame. Return the pasta to the cooking pot. Add 2/3 of the clam juice, plus the liquid that has collected in the bowl of Manilla clams. Add 3 tablespoons of the minced parsley. Stir to blend, about one minute. Divide pasta among 5 to 6 individual pasta bowls and top with the remaining clam juice, the remaining parsley, and the Manilla clams. Serve immediately.