Hi,
Unfortuantely this is not possible. The main reason is, that the WordPress results page layout is completely controlled by the currently active theme you are using. To group results in any way, the complete layout has to be custom coded, but even then it is not the greatest idea as it requires custom queries from existing results. I have only seen a very few cases where something like that worked.
Instead of trying to custom code the results page, you could try this alternative solution. That will force the search shortcode as a results page container on a custom page. It is far from perfect, but it does not require coding.