The deck clutch belt is connects to the clutch and a pulley attached to a spindle housing/driveshaft. If the spindle housing hasn't been greased the bearings could be bad and causing play to wear the belt out. Spring tension could be a reason also. If both belts keep going out I would check the clutch bolt to see if the clutch and pulley are moving around on the engine shaft.
It's pretty simple design. Guessing if a good mechanic takes a look at it they would figure it out quickly.