In a medium saucepan, bring the clam juice and water to a boil. Cover and keep hot over low heat. In another medium saucepan, heat the remaining 1 tablespoon of olive oil. Add the onion and garlic and cook over low heat, stirring occasionally, until softened, about 4 minutes. Add the rice and stir over moderate heat until coated with the oil. Add the wine and saffron and simmer until almost evaporated. Add enough of the hot clam broth to just cover the rice and cook, stirring, until the broth is almost absorbed. Repeat until the rice is al dente and bound in a creamy sauce, about 25 minutes. Keep the remaining 1/2 cup of broth hot.