Since you have a SDS for your Scout, I would suggest replacing the VPI belts with a thread belt. I tried mono-filament fishing line (Spiderwire) and silk thread. I preferred the silk thread using a blood knot as I can get the knot to reduce to a smaller size.
The Scout motor pulley is tapered on both the 600 or 300 RPM motors. You can verify this on yours by using one belt and setting the speed, then move the belt to another pulley grove. If the speed changes, the pulley is tapered.
If I were to use two belts, I would ensure both are talcum powdered so I did not have too much noise introduced by buckling or slipping due to the different diameters of the pulley groves.
The Scout motor pulley is tapered on both the 600 or 300 RPM motors. You can verify this on yours by using one belt and setting the speed, then move the belt to another pulley grove. If the speed changes, the pulley is tapered.
If I were to use two belts, I would ensure both are talcum powdered so I did not have too much noise introduced by buckling or slipping due to the different diameters of the pulley groves.