Redirecting mobile users to another theme

Hi everyone,
I want to make my theme redirecting mobile visitors to another theme. I found a lot of tutorials either using a bunch of plugins (which I don’t want to) or telling me, that you cannot redirect outside of a plugin. I found this way to change the stylesheet and template directory but not to change the folder of the themes:

add_filter('template', array(&$this, 'switch_theme'));
add_filter('option_template', array(&$this, 'switch_theme'));
add_filter('option_stylesheet', array(&$this, 'switch_theme'));

function switch_theme()
{
global $device;
return $device;
}

Using this in my themes functions.php it changes the stylesheet directories but not the directory for the files like head.php or index.php to the other folder. Where am I wrong? Any help would be appreciated!!!

Thanx in advance
Dietmar