Assuming you're writing actually detailed and complete user stories and those user stories are for functionality that basically amounts to "just math" (i.e. no UI or API standards or whatever to worry about alongside the business logic) I imagine it's not much more difficult or time consuming to just write the code rather than describing it in a user story.

Having one person write user stories and a second person, who understands the domain less than the first person write the actual code would be twice as much work for a worse result.