In a medium, heavy saucepan, melt oil and 2 tablespoons of butter over medium heat. Add the leeks and saute until tender but not brown, 5-6 minutes. Add the rice and stir to coat with the butter. Add the wine and simmer, stirring, until the wine has evaporated, about 3 minutes. Add a ladle-full (about 1/2 cup) of the hot broth and stir until almost completely absorbed, about 2 minutes. Continue cooking the rice, adding the broth 1/2 cup at a time, stirring constantly and allowing each addition of the broth to absorb before adding the next, until the rice is tender but still firm to the bite and the mixture is creamy, 15-20 minutes. Remove from the heat. Stir in the remaining tablespoon of butter, Parmesan, mascarpone cheese, the lemon zest and juice, and season with salt and pepper to taste. If rice thickens while you prepare the scallops, add a bit more hot broth or water just before serving.