add another variable to php function

Maybe a late reply, but you can achieve this by using Logical Operators (see https://www.php.net/manual/en/language.operators.logical.php for more information about this subject)

So let's change the if statement to add the second slider

// orderby slideshows
function modify_slider_order($query, $slider_id) {

    if($slider_id == 17 || $slider_id == 16 ) {

        // Custom Meta key/name
        $query['meta_key'] = 'numeric_order';

        // Order by Custom Meta values
        $query['orderby'] = 'numeric_order';

        // Calculate order based on:
        // 'NUMERIC', 'CHAR', 'DATE', 'DATETIME', 'TIME'
        $query['meta_type'] = 'NUMERIC';

    }

    return $query;

}
add_filter('revslider_get_posts', 'modify_slider_order', 10, 2);

See what we did there? || is a logical operator for OR. So the function will execute if the ID equals 17 or 16.

Well I figured out I could duplicate the function and rename the title so it runs twice. But for future, is there a more elegant way of doing this in one instance? thanks!

// orderby slideshows

function modify_slider_order($query, $slider_id) {

// only alter the order for slider with "x" ID
// https://tinyurl.com/zb6hzpc

if($slider_id == 17 ) {

    // Custom Meta key/name
    $query['meta_key'] = 'numeric_order';

    // Order by Custom Meta values
    $query['orderby'] = 'numeric_order';

    // Calculate order based on:
    // 'NUMERIC', 'CHAR', 'DATE', 'DATETIME', 'TIME'
    $query['meta_type'] = 'NUMERIC';

}


return $query;

}

add_filter('revslider_get_posts', 'modify_slider_order', 10, 2);

// orderby slideshows2

function modify_slider_order2($query, $slider_id) {

// only alter the order for slider with "x" ID
// https://tinyurl.com/zb6hzpc

if($slider_id == 16 ) {

    // Custom Meta key/name
    $query['meta_key'] = 'numeric_order';

    // Order by Custom Meta values
    $query['orderby'] = 'numeric_order';

    // Calculate order based on:
    // 'NUMERIC', 'CHAR', 'DATE', 'DATETIME', 'TIME'
    $query['meta_type'] = 'NUMERIC';

}


return $query;

}

add_filter('revslider_get_posts', 'modify_slider_order2', 10, 2);