I've installed the Better Together files meticulously and followed the instructions. My problem is that my custom skin - clean_business doesn't have a /templates/ folder in it.
So I created one then copied the template_default/templates/tpl_product_info_display.php as well as the tpl_better_together_marketing.php file into it. So now they are both in both places with the customized code according to your instructions.
Yet, it's still not working - I've gone all the way to the checkout page - but it's not there.