In a saucepan, bring 2 cups water to a boil. Stir in the couscous and 1/4 teaspoon salt. Cover and let sit off heat for 5 minutes; fluff with a fork before serving.
Meanwhile, heat 1 tablespoon of the butter in a large skillet over medium-high heat. Add the garlic and scallions and cook for 30 seconds. Add the shrimp and cook, stirring, until they begin to turn pink, about 3 minutes.
Stir in the beans, parsley, lemon juice, the remaining butter, 1 teaspoon salt, and 1/4 teaspoon pepper. Cook until heated through, 2 to 3 minutes. Serve with the couscous.