From a SEO perspective this would qualify as double content and you could get penalized by search engines for it unless you specify the right canonical URL or something, but the best practice would be to redirect one of the URL’s to the other. Whichever you consider the most important.
To assist you with your question; You can make custom menu stuctures (in the Appearance > Menus section). That way you can include the same page in a menu structure as much as you want without having to create duplicate pages. And quite frankly I wouldn’t use a different URL for purposes mentioned earlier.
EDIT:
To be able to edit how the URL is formed you could edit the url of the parent page (product-pages) to anything you want. I believe any child pages you create will carry the same parent name. But if you really want to turn the URL forming around, you should look into creating custom post types, where you have the most freedom of creating a URL that suits you.