1. Carriers do NOT wear out.
2. It is MOST likely they are both on the same carrier. IF they are back to back it would be impossible to install two carriers unless the wall is about 18" thick.
3. There are only "carriers", so it was NOT an American Standard carrier. It was a carrier with an American Standard toilet mounted on it. Whether an inwall tank would fit above it would depend on how it is made and you would have to open the wall to determine that, although it will probably fit.