Position a rack in the center of the oven and preheat to 425 degrees . In an extra-large skillet, cook half of the pancetta slices, undisturbed, over medium heat for 3 minutes. Transfer to a plate and repeat with the remaining pancetta slices.
Heat 2 tablespoons olive oil in the same skillet over medium heat. Season the tenderloin halves with salt and pepper, add to the skillet and cook, turning, until browned, 6 to 8 minutes. Transfer to a work surface and let cool.
In the same skillet, cook the chopped pancetta in the remaining 1 tablespoon olive oil, stirring, until golden, 5 minutes. Add the butter to melt and transfer to a medium bowl. Add 1/4 cup water to the skillet and cook, scraping up any browned bits; pour into the bowl along with the breadcrumbs, parsley, thyme and 1/2 teaspoon each salt and pepper. Reserve the skillet.
Arrange four 12-inch-long pieces of kitchen twine 2 inches apart on a cutting board, parallel to the front edge; place 1 tenderloin half in the center, perpendicular to the strings. Slice the tenderloin lengthwise, three-quarters of the way through. Open the meat like a book; working from the center out, slice each half of the book lengthwise again, three-quarters of the way through, opening outward, so the split tenderloin has 4 attached panels. Spread half of the breadcrumb stuffing across the meat, leaving a 1-inch border. Starting at a short end, roll up the meat; overlap half of the reserved pancetta slices on top, then secure with the twine. Transfer the meat to a large roasting pan. Repeat with the remaining tenderloin half, stuffing and pancetta.
Roast the tenderloins until they register 120 degrees on an instant-read thermometer for medium-rare, 25 minutes. Transfer to a work surface and tent with foil. Pour the pan juices into the reserved skillet, add the mushrooms, season with salt and pepper and cook over medium-high heat until browned, 10 minutes. Stir in the port for 1 minute. Slice the roasts and spoon the mushroom sauce on top.