creating a second image attachment template?

attachment.php is not the only template file for attachment display. mime-type based template files get priority.

• MIME_type.php – it can be any MIME type (image.php, video.php,
  application.php). For text/plain, in order:
  1. text.php
  2. plain.php
  3. text_plain.php

http://codex.wordpress.org/Template_Hierarchy#Attachment_display

I expect that that is the issue you are seeing. Check your theme for image.php.

There is a further problem with your code. Attachments don’t have categories as far as I know. The category is inherited from the parent post. That means that you will need to pass the parent post ID to in_category.

if ( in_category('2', $post->post_parent) ) {
   get_template_part('bet-photo'); 
}

I would also suggest get_template_part, as in my version of the code, instead of PHP’s include.