Hint: Register with JD Parts and you get individual part specifications. With proper alignment and adjustment an OEM set would last a long time. Typically tandem belt sets are "matched" for best service life meaning replace them both at the same time with the same brand so that they equally share the power transmission.
Here is the 400 PTO belt spec cut and pasted from JD Parts:
Specification
Weight: 0.62 LBS 0.28 Kg
Belt Section HA
Effective Length 41.270 IN
Number of Belts 2.000
Width 0.550 IN
Thickness 0.310 IN
Matched Set 2HA
Angle 32
Material Spec
Finish
Note D
Here is the 400 PTO belt spec cut and pasted from JD Parts:
Specification
Weight: 0.62 LBS 0.28 Kg
Belt Section HA
Effective Length 41.270 IN
Number of Belts 2.000
Width 0.550 IN
Thickness 0.310 IN
Matched Set 2HA
Angle 32
Material Spec
Finish
Note D