Did you tried "Copy item" then "Delete item" action ?
It would copy the completed task to your destination task list, then remove the original one.
Please do create a new [Thread] / [Topic] with your question and requirement.
I am sure we can help you out ... in most cases at least 🙂
ps: yes, you need two seperate: one is the original list and second one will act as the back up. The workflow then will copy items from the original list to the back up list.