Toss apples in bowl with cider and vinegar. Cut each pork tenderloin crosswise into 6 pieces. Place between 2 pieces of plastic wrap. Pound each piece to 1/3 inch thickness. Sprinkle with salt and pepper.
Heat 2 T oil in heavy large skillet over med-high. Lay 6 sage leaves in bottom of skillet. Place 1 pork medallion on each leaf, pressing down lightly with back of spatula.
Reduce heat to medium; cook until underside of pork is golden, about 3 minute Place 1 sage leaf on top of each medallion and carefully turn each piece to keep sage in place. Cook til pork is just done, about 2 minute.
Carefully transfer pork to plate and keep warm. Repeat with remaining pork, sage and oil. After removing pork from skillet, add apple mixture to skillet; cook until liquid is reduced by half, and apples are almost tender, about 5 minute.
Return pork to skillet along with any juices that have accumulated; cook til heated through, about 2 minute Using slotted spoon, transfer pork and apples to serving platter. Add butter to pan; stir just til melted. Season with salt and pepper; pour over pork and serve.