You would have to create your own index or customize the results somehow.
You can modify /CMSModules/SmartSearch/Controls/SearchResults.ascx to customize your result. There is a dataset that contains all the values for your search results, you'll need to remove all the sub pages from the dataset and add the necessary parent pages into the dataset. I haven't tried this approach before, so you'll have to tell me if it works.
1) Create two indexes, one that is normal and the other for the sub pages
2) Add 2 search results onto the page, one for each index. Make sure the sub pages search results is executed first.
3) set the sub pages search results to display:none or visible=false in code behind but only after code executes.
4) When the code executes for the sub page search results, store the values of all the unique parent node ids for the sub pages, and pass it to the normal search results control.
5) When the code executes for the normal search results, retrieve all the content for those ids captured in (4), and add them to the data set in the search results control.
6) Wait and see.