Please do not offer intermixed results, at least offer a way to switch them off. Editors often rely on search for adding internal links (e.g. I wrote an article, which articles should link to it?) or fixing errors (e.g. I fixed a typo, what other articles must have the same typo). The best approach is not offer intermixed results at all, but offer results for sister projects separately instead.
Topic on Talk:Cross-wiki Search Result Improvements/Design
Appearance
We'll have a function to 'turn off' the cross-wiki search results by project/language, but it sounds like you'd want that ability within the browser itself. Is that correct?
Perhaps yes. For instance, if Italian Wikipedia has chosen intermixed results and I am looking for something explicitly in Italian Wikipedia and definitely not in any sister projects, I want to have a possibility to filter the results this way.
This means that I don't want to change the decision of Italian community if I need this feature for only one search.
Thanks for the clarification!