Currently, you can check both selected and excluded values selected in the same filter. However, only the last tab of select/excluded you viewed is applied. For instance: If I want to see all tickets with closed status, and exclude pending, and I try do this from the same filter, only one of those will be applied, based on whichever I set last. Given that if I re-open the filter, I will see ALL previous values in both I checked saved and showing as such, it is incredibly misleading.
You can try this out by setting a selected and excluded value in a filter, and then toggling between which one you hit apply on. You'll see your values change depending on which one was last, and the other tab value completely ignored.
If I can't actually select options for both and have them apply, the system should not allow me to choose options in both sections.