In small bowl, stir together sour cream and whipping cream. Cover and let stand at room temperature for 2 hours. (Mixture will thicken as it stands.)
Meanwhile, for stuffing, in medium skillet, cook shallot in 1 tablespoon butter over medium heat for 2 minutes; remove from heat. Stir in apple, raisins, bread crumbs, walnuts, egg, and parsley; set aside.
Trim fat from meat. To butterfly each tenderloin, make a lengthwise cut down center of meat, cutting to within 1/2 inch of the other side, but not through it. Starting at the center of the meat, make one horizontal slit to the right, cutting to within 1/2 inch of the other side. Repeat on the left side of center. Cover with plastic wrap. Use a meat mallet to pound each piece of meat into an 11x6-inch rectangle. remove plastic wrap.
Spread half of the stuffing over each rectangle. Starting from a long side, roll up into a spiral. Tuck ends under and place, seam sides down, on a rack in a shallow roasting pan. Brush with the melted butter and sprinkle with the herbs.
Roast, uncovered, in a 350 degree F oven about 50 minutes or until instant-read thermometer inserted in meat registers 160 degrees F. Transfer to a serving platter. Cover with foil; let stand for 5 minutes before slicing. (The meat's temperature will rise 5 degrees F during standing.)
For sauce, stir apple cider into roasting pan, scraping up browned bits. Place sour cream mixture in a small saucepan. Gradually stir cider mixture into sour cream mixture; heat through. To serve, cut meat into slices and spoon sauce over meat. Makes 6 to 8 servings.