@kj93 

Well from what you are saying you need a flow triggered by placed order metric and then use trigger splits to check who bought what.. so for example you need a trigger split and in it you can check for products or collections (it would be much easier if you have all of these sorted in collections) so for example in this specific situations you should have 3 trigger splits.. 

• people who bought both tops and skirts and decide how to address them
• people who bought just skirts - sell them tops
• people who bought tops - sell them skirts

But again for this to be possible you need to be sure that in your store you have those 2 collections (skirts and tops). Good thing is the collections in the store don’t have to be public and part of a website menu and they will still show up in klaviyo.