Bring the shrimp broth and the chicken broth to a gentle simmer in separate saucepans. In a large saucepan, cook the onion and garlic in the 2 tablespoons olive oil over medium heat, stirring every minute or so, until the onion turns translucent but doesn’t brown, about 8 minutes. Add the rice and stir over medium heat for about 2 minutes. Add a cup of the shrimp broth and stir for 1 minute, or until the broth disappears. Add another cup and stir again; continue in this way until you’ve used all the shrimp broth, and then start using the chicken broth. After 15 minutes, start nibbling on a grain of rice to check for doneness—there should be just the slightest resistance—and add only ½ cup of liquid at a time so you don’t add too much and make the rice too runny. The consistency can range from almost as stiff as a pilaf to the consistency of loose baked beans, but you should keep the risotto slightly on the stiff side because when you add the butter and olive oil it’s going to get runnier. When the rice is done, usually after about 25 minutes, stir in the saffron, remaining olive oil, butter, and shrimp. If the risotto is too stiff, add just enough hot broth or water to give it the consistency you like. Season to taste with salt and pepper. Serve on hot plates or soup plates.