How to insert a page between multiple pdf files when combining said pdf files?

I combine multiple invoices into a single large PDF file to upload to my accounting system and am wondering how to insert my invoice seperator page between the individual invoices. After searching Google, I can only find how to insert my separator page after each page of an invoice. The problem is that some of my invoices have multiple pages, which when running the action wizard, my separator page will be inserted after every single page of my invoices regardless if the invoice is one page or multiple.

I'd like to have the separator page be inserted between each PDF file and not each and every page. Before I was just manually dragging and dropping the separator between each pdf file when combining. I'm hoping there is a solution that saves time.