Currently, to me, it looks like you are manually creating a Defect record after manually creating a story. This process can be automated, and there are a few ways you can go about this... one would be through Flow Designer. Trigger: story is created. Actions: create a Defect record and set value of Tester to the story's Tester.
Your other option (requires some custom scripting) would be to create an After Insert business rule that gets triggered when a Story is created. In the script, you'll use a GlideRecord query to create a new record on the Defect table, and set the current (Story) tester equal to the Defect tester.
Flow designer is probably the easiest way to accomplish this. Here's a link to SN docs. I would also recommend creating a reference field on the Defect table that points to Story (if that doesn't already exist). This way, you can pull that column onto the Defect record as well, and have a link back to your original Story record.
