meta_query sorting by 2 keys

OK, the final workaround would be to split query:

$sfp_query_args = array(
    'tax_query' => array( array( 'taxonomy' => 'sfp_post_category', 'terms' => $cat_id_arr ) ),
    'meta_key' => 'is_sponsored',
    'post_type' => 'sfpposts',
    'post_status' => 'publish',
    'showposts' => (int)$per_page,
    'paged' => $paged
);

…and use “posts_orderby” filter to modify ORDER part:

add_filter( 'posts_orderby', 'sfp_modify_orderby' );
function sfp_modify_orderby( $orderby ) {
    if( !is_admin() && is_tax( 'sfp_post_category' ) ) {
        global $wpdb;
        $orderby = " $wpdb->postmeta.meta_value DESC, $wpdb->posts.post_date DESC ";
    }
    return $orderby;
}

The most probably you’ll need to remove filter after the loop on page in order to prevent ‘posts_orderby’ affect any other query (sidebar or footer). So here’s another function to put in “functions.php”:

function sfp_remove_orderby_filter() {
    remove_filter( 'posts_orderby', 'sfp_modify_orderby' );
}

…and on the page using our query discard filter:

if( have_posts() ) : while( have_posts() ) : the_post();
    // code
endwhile;
else :
    // code
endif;

sfp_remove_orderby_filter();

Hopefully it makes sense!