Rewrite /keyword1+keyword2.html to search page | .htaccess

Don’t touch the .htaccess file, there’s this neat tool add_rewrite_rule() which you should hook into init:

add_action('init', function() {

    add_rewrite_rule(
        'search+([^/]+).html$',
        'index.php?my_custom_search=1&s=$matches[1]&submit=Search',
        'top'
    );

});

Right after that you should flush the rewrite rules once ( by saving the permalink settings in Settings > Permalinks or with flush_rewrite_rules() BUT call that function only once and when needed ).

This should make www.site.com/search+keyword.html serve search results as www.site.com/?s=search+keyword&submit=Search