Something like this should work for you:
RewriteEngine On
RewriteCond %{HTTP_REFERER} !^http://example.com [NC]
RewriteRule ^form/page2.php$ http://example..com/form/page1.php [L,R]
Going to http://example.com/form/page2.php
will redirect you to http://example.com/form/page1.php
unless you are linked to page2.php
from within site.com
.