In a medium saucepan, combine the clam broth and water and bring to a simmer. Keep warm.
In a large saucepan, heat the olive oil. Add the onion, season with salt and pepper and cook over moderate heat, stirring, until softened, about 5 minutes. Add the rice and cook for 1 minute, stirring to thoroughly coat. Crumble the saffron into the wine and add it to the rice. Cook, stirring until the wine is absorbed. Add 1 cup of the warm clam juice and cook over moderate heat, stirring constantly, until nearly absorbed. Continue adding the juice 1/2 cup at a time, and stirring constantly until it is nearly absorbed between additions. The risotto is done when the rice is al dente and suspended in a thick, creamy sauce, about 20 minutes total. Season with salt and pepper.
Melt the butter in a large skillet. Add the shallot and cook over moderate heat until softened, about 2 minutes. Add the shrimp and crab and cook until just heated through. Scrape the seafood into the risotto and stir in the parsley and mascarpone. Serve immediately.