PREPARATION IN Part-Editor
STEP 1: Edit the Base-Part
STEP 2: Add a different sketch-loop to represent the Path of the second Belt
You must merge the center-Point of the Arc for the second belt, with the center-Point of the Arc for the first Belt.
(Add the Arc first, then use Merge-Point tool in the Geometry toolbar).
STEP 3: You MUST add a dimension to each Arc
The radius of the Arc is not important. You can edit the number-of-teeth on Pulley later.
See Pulley dialog-box
STEP 4: Close the Part-Editor