In resealable bag, mix together 1/2 cup balsamic vinegar, olive oil, rosemary, garlic, salt, and pepper.
Add the two pork tenderloin pieces and seal bag. Place on counter top for one hour, turning occasionally.
Preheat oven to 325 and spray cookie sheet with non stick cooking spray.
Place tenderloins on cookie sheet and roast about 45 minutes or until nearly done.
Remove from oven and wrap in foil to rest. Meanwhile, boil remaining 1/3 cup vinegar until reduced by half, about 8 minutes.
When vinegar is reduced, remove pork from foil and slice about 3/8" thick. Drizzle reduced vinegar over slices and serve.