Not that I know of (Low or SolSpace may chime in if I'm wrong about their modules capacities), however you could do some "magic" on the results template. I'm thinking if you just do a content/entries search, then on your results page you could do a manual/PHP user search (as users have limited fields!) on the results template, appending these results as needed.
By example, something like this :
<?php
//get the search term
$searchTerm = ee()->uri->segment(3);
//here split the actual search term from the search string
//Now search the members table
$searchedUsers = array();
//search username, screen name and email...
$query = ee()->db->like('user_name',$searchTerm)
->or_like('screenname',$searchTerm)
->or_like('email',$searchTerm)
->get('members');
foreach ( $query->result_array() as $row ) {
$searchedUsers[] = $row;
}
?>
{!-- here we display the channel entries results --}
{exp:low_search:results query="{segment_3}" limit="10"}
{if count == 1}<ol>{/if}
<li>{title}</li>
{if count == total_results}</ol>{/if}
{if no_results}No search results{/if}
{/exp:low_search:results}
{!-- And now the member results --}
<?php
if ( !empty($searchedUsers) {
echo "<ol>";
foreach ( $searchedUsers as $sUser ) {
echo "<li>".$sUser['screenname']." - ".$sUser['email']."</li>";
}
echo "</ol>";
}
?>
(you'll probably need PHP on Output so you can parse the search tags and get the search term)
By example, absolutely untested but should be a good starting point!
After thought - please read
Now I think about it, doesn't SolSpace User use a channel to store users data? I could be wrong but maybe you just need to identify the User Channel and feed that into Low Search? Hopefully SolSpace will pop in and confirm if this works (They're regularly active so shouldn't be too long ;) )...
Scrap this, I was thinking of another module