Yes u can perform this. If u know the basic spd list workflow then try this---
Whenever you are updating or using an item of some other list (on which the workflow is not running), then you need to have a column in that list whose value will match with the column of the item, on which the workflow is running. Now in your case, both the lists (list1 and list2) must have a common column value for each list item. This is required because you need to tell the workflow that out of several items on the other list, which item he should update. Therefore, for that you need to use a column of an item in the other list whose value equals any of the column value of the item on which the workflow is running. In short, for unique identification. By using this u can get value of list2(assigned quantity) and minus it from list1 (qauntity field) and save back it to list1.
Hope it helps