Hello There,
Thanks for writing in! To do what you have in mind, you can make use of the one page navigation menu feature present in Page Settings. You need to create two separate menu. One menu should be assign as your primary menu. The second menu will be use for your one page navigation menu. Now your second menu should be created intended for pages only. And then you’ll have to edit each of your pages and assign the second menu in the Page Settings taking advantage of the One Page Navigation menu feature. This should work even if your pages is not a one page navigation. The second menu will exclusively for the pages and your single post will have the primary menu.
Hope this make sense. Please let me know how it goes.