In a medium saucepan, boil 1 cup broth until reduced to half a cup, about 15 minutes.
Season 2 pork tenderloins with ¼ teaspoon salt and ⅛ teaspoon black pepper, or to taste.
Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Add pork and sear (quick browning while the inside is still raw) well on all sides. Remove and reserve on a plate with the juices that might've collected.
In the same skillet, heat the remaining 2 tablespoons olive oil over medium heat.
Add ½ cup sliced shallots, some black pepper to taste, and cook over medium heat until just beginning to color, about 3 minutes.
Add ¾ cup Port wine¼ cup Port wine and reduce until almost evaporated. Add ¼ cup of the reduced stock and let reduce again until almost evaporated. Repeat with another ¼ cup Port.
When the mixture is thick and the liquids are syrupy, add the seared pork back to the skillet and the remaining ½ cup broth and ¼ cup of Port.
Let cook for about 15 minutes, turning frequently. This depends on the size of your tenderloins and how cooked you like them. You can also finish cooking them in the oven.
Remove from heat, transfer pork to a cutting board and let rest for 2 or 3 minutes to redistribute the pork's juices.
Meanwhile, check the remaining sauce left in the skillet. If you feel it's too liquid reduce it a bit more while the pork is resting.
Cut the meat into inch-thick slices, arrange on the plates, and spoon sauce over. Place sweet potatoes on the side.