you need to have the record selection for both years, but do not put the fields themselves in the detail section
 
instead, create a formula, such that, "if part date in period lastyearYTD then part quantity field else 0" and same for the current year formula " if part date field in period currentyearYTD then part quantity field else o".
 
then place those in the detail section. create a group on your part number and subtotal your formulas.
 
there are other ways as well, depends on the complexity being created.
 
hth