i really don’t know if there is a solution for that.
WordPress requires a unique slug for each category term – For example when you create:
The same page should be reachable with just:
Which is why duplicated category terms have to have different slugs.
The only solution i can think of is to NOT use a sub-category but instead use another taxonomy or attribute for that. Which could be common to all products. But then you may require a developer to create some rewrite rules for the slugs.