Search in multiple forums with pagination

What sometimes helps in answering API questions is taking a look at the page URL when using the community search page and applying some filters.
I couldn't find a way both there or VIA API to filter by multiple locations. So probably you need to work around a bit like that (Pseudocode):

if (number_resultsperpage_from_firstboard => results_per_page) {
// If the results from the first board already fill a page only show those
} else {
//on the page first show the results from the first board and then have another API call to pull results from second board to fill up the page with parameter page_size = results_per_page - number_resultsperpage_from_firstboard
}

 i know that's very rough because it does not include all of the pagination checking, but I hope you get the general idea.

At least I can reply the second part of the question:
On the "Search" page of your community click on "Advanced Search" and you will see the three parameters as input fields together with small (i) icons with a more detailed explanation.