In each of 2 medium saucepans, heat 2 tablespoons butter over medium-low heat. Add half of the onion and garlic to each pan and cook, stirring frequently, until softened, about 7 minutes. Add the corn to one pan, and add the roasted red peppers with their brine along with the croutons and cumin to the other pan. Add 1 can chicken broth to each pan and bring to a boil. Reduce the heat and simmer both soups, stirring occasionally, for 20 minutes. Stir the sour cream into the corn soup.