You need to use pre_get_posts hook. Where u can include the default category posts on other category pages.
function wpse_pre_get_posts( $q )
{
// its the main query, and its the category page archive
if( $q->is_main_query() && $q->is_category() ) :
// get the default selected category id, fallback to 1 (default one id)
$default_cat = (int) get_option('default_category', 1);
// if the category page is not the default category page
if( $default_cat != $q->get_queried_object_id() ):
// get the already used tax_query arguments
$tax_query = $q->get('tax_query');
// add the default one
$tax_query[] = array(
'taxonomy' => 'category',
'field' => 'term_id',
'terms' => $default_cat
);
// relation should be or, which means either the current category or the default
$tax_query['relation'] = 'OR';
// set the values to query
$q->set('tax_query', $tax_query );
endif;
endif;
}
add_action( 'pre_get_posts', 'wpse_pre_get_posts' );
That's it. You are all done