If you implement this, please make sure any auto-refocus has visual clues, e.g. looking like a single input field, and behaving like a single field (a TAB moves to the next logical form field).
When inputting license keys in programs, and there were dashes in them, it was always fun encountering an auto-focus across split textfields where you didn't expect it, so you accidentally skipped a field and had to redo it.
When inputting license keys in programs, and there were dashes in them, it was always fun encountering an auto-focus across split textfields where you didn't expect it, so you accidentally skipped a field and had to redo it.