How do I use 2 different attachment.php pages?

I don’t know of a way to have two (or more) attachment templates, but if there is something in these “posts” which can act as a trigger, you can use that to modify the template to suit.

For example, let’s say you insert an HTML comment like:

<!--ad-->

in the description of an image attachment. A small bit of PHP can look for it in the description (technically, the post_content field of the attachment):

<?php
if( strstr('<!--ad-->',$post->post_content) ) :
?>
~ attachment advert stuff goes here ~
<?php endif; ?>

Note something like this is aimed at running within The Loop, where the $post object is put to work.