Ricotta cheese gives twice-baked potatoes a creamy taste without adding fat.
Heat oven to 450 degrees. Bake potatoes until tender, 50 to 60 minutes. Make a cut lengthwise along top of each about four inches long. Using a melon baller, remove flesh of each potato; transfer to medium bowl. Reserve skins. Pass potatoes through a ricer or food mill into a medium bowl.
Heat oil in a nonstick skillet over medium-low heat. Add garlic and shallots; saute, stirring frequently, 6 to 8 minutes. Stir half the garlic and shallots into riced potatoes. Set remaining half aside in skillet.
In a food processor, puree ricotta until smooth; mix into riced potatoes along with milk, 1 teaspoon salt, and 1/8 teaspoon pepper. Fill potato skins with cheese-and-potato mixture until brimming over edges. Transfer to baking sheet; place in oven until heated through, about 15 minutes.
Return remaining garlic and shallots to medium heat. Add tomatoes, remaining 1/2 teaspoon salt, and 1/8 teaspoon pepper; cook, tossing until tomatoes are warm, about 3 minutes. Stir in 1/2 tablespoon marjoram; cook 1 minute. Spoon 3 tablespoons tomato mixture over each potato; garnish with remaining 1/2 tablespoon marjoram.