If you already have 1 Amazon listing linked to a Product and would like to link more Amazon listings to this Product, you will want to follow these 4 steps...
1) Duplicate your product
Create a temporary product in Sellbrite by duplicating your first one, but with a different SKU (we'll delete this product soon so it can be anything).
2) Build your new listing
Build your listing to Amazon from this newly created temporary product in Sellbrite.
3) Delete the Temporary Product
Now that your Amazon listing is "active", you can delete the temporary product you created during Step 1, which will cause the Amazon listing created during Step 2 to become unlinked.
4) Re-link your new listing
Link the Amazon listing created during Step 2 to your original Product in Sellbrite.
You will now have 2 Amazon listings linked to 1 Product in Sellbrite.
Why do this?
The reason we need follow this process and not just create a new listing to Amazon from our already linked product in Sellbrite is because Amazon requires every SKU/ASIN combo to be unique. So, we need to create the listing to Amazon from this temporary SKU in Sellbrite, then unlink the listing from the temporary product so we can link it to the appropriate product for inventory management.
NOTE: You can also create the listing in Amazon first with a different SKU, then import the listing into Sellbrite to link to your 1 Product.