Melt butter in a large pot over medium heat. Cook and stir onion in hot butter until softened, about 3 minutes. Crumble pork sausage and spicy pork sausage into onion; cook and stir until sausage is browned and cooked through and onions are tender, 7 to 10 minutes.

Stir flour, salt, and black pepper into sausage mixture, adding additional butter if mixture is too dry, and cook until flour begins to brown, about 5 minutes.

Pour milk into sausage mixture, increase heat to high, and bring to a boil, stirring constantly, adding more milk if mixture is too thick. Reduce heat to low, cover the pot with a lid, and simmer, stirring occasionally, until flavors blend, about 20 minutes.