Preheat the oven to 375 degrees F and butter a 3-quart baking dish. Melt 6 tablespoons butter in a large pot or Dutch oven over medium-high heat. Add the bacon and cook, stirring occasionally, until browned and crisp, about 7 minutes. Add the onions and cook, stirring occasionally, until very soft and caramelized, about 20 minutes. Add the celery, thyme, 1/2 teaspoon salt and a few grinds of pepper and cook, stirring occasionally, until the celery is tender, about 5 minutes. Add the chicken broth and bring to a simmer, then remove from the heat and set aside.
Whisk the eggs, parsley and scallions in a large bowl. Add the cornbread and potato bread cubes, the pecans and the vegetable mixture; stir to combine. Transfer the stuffing to the prepared baking dish. Cut the remaining 2 tablespoons butter into cubes and dot over the stuffing. Cover with foil and bake 30 minutes, then uncover and bake until golden brown, about 20 more minutes.