explicit "switch to mobile theme" link with Theme Key
One way would be to nest each theme switching rule inside another rule using

system:cookie = theme=theme1

     Theme switching rule here


system:cookie = theme=theme2

     Another theme switching rule here


You will need to set up your theme switching links to create a cookie using your method of choice, PHP or jquery.cookie

An example using jquery.cookie could be something like:

template.php:

function THEMENAME_preprocess_page(&amp;$vars) {
  drupal_add_library('system', 'jquery.cookie');
}


Your theme switching links:

&lt;a href="#" onclick="jQuery.cookie('theme', 'theme1'); location.reload(); return false;"&gt;Switch to Theme1&lt;/a&gt;
&lt;a href="#" onclick="jQuery.cookie('theme', 'theme2'); location.reload(); return false;"&gt;Switch to Theme2&lt;/a&gt;


Keep in mind that this is not page caching friendly.  If you want this to work for anonymous users, you will need to disable page caching.