How do you want to see the found set?
In portal, you can't "find" with additional criteria. Find is done on base table of layout. To decrease found set in portal, you need additional "relation condition" or "portal filter".
In list view, you can use layout based on any of "base table" or "TO" in "Go to related records". So "base table" may be used usually, so that no need to make another layout on "TO", or can "Go to related records" again on the layout.