Pour the chicken broth and water into a saucepan, stir in the garlic, and bring to a boil. Stir in the couscous, cover the pan, and remove from heat. Allow the couscous to stand until all the water has been absorbed, about 5 minutes; fluff with a fork. Allow the couscous to cool to warm temperature.
In a large serving bowl, lightly toss the couscous, sun-dried tomatoes, olives, feta cheese, and garbanzo beans. Mix the oregano, black pepper, white wine vinegar, and lemon juice in a small bowl, and pour over the couscous mixture. Toss again to serve.