what model is it??? my FIL has a 21XX (cant remember the exact model) Cub Cadet with two belts on the ~50" deck (one on the deck and one to go to the engine) a few weeks ago it shredded the deck belt, the cause was the tensioned freezing up, it was bolted to the deck, no double nutting or anything, I replaced the bolt with a longer one, put on 2 washers above and below the tensioner arm and double nutted the bolt that holds everything together. it should not have that problem again.
Aaron Z