This is common when your domain is on something like www.yourdomain.com, but your shopping cart is located elsewhere, like www.ecommercestore.com/store
As long as you have the ability to add code to both pages, you can track everything with the same tag.
Let's say you want to track a landing page at www.yourdomain.com/store/sales.html.
Make sure the tracking tag is installed on the "sales.html" page and create a Retargeting Audience to point to that page:
Now we have a Retargeting Audience that will capture users who visit your sales.html page. You also want to track conversions - in this case, when a purchase is made. But your shopping cart/thank you page is on another domain! Specifically, it's at http://www.ecommercestore.com/store/cart/thankyou.html
No problem. Just make sure the tracking tag is installed on your thank you page, and create the conversion goal to point to that path:
Our tracking tag ignores the domain name and .com and will only pay attention to what comes after.
So, as long as you've got the tracking tag installed on both domains and have your paths setup correctly, you can track Retargeting Audiences and conversion goals across multiple domains!