Both output taxa need to be atlased:
We’ve made some changes that allow curators the flexibility, if they choose, to make Taxon Splits where the input taxon is also one of the output taxa. In some ways, this approach is analogous to using a taxon swap to broaden an existing taxon (e.g. Species X) instead of using a taxon merge with a new output taxon (e.g. Species X’) as was described above. In this approach, the input taxon is first manually narrowed by modifying the atlas (as well as other content such as range map, etc.) which will be used to determine the destination of identifications.
Upon committing such a Taxon Split, iNaturalist will not inactivate the input taxon. It will still use the atlases to determine which identifications should be updated with which output taxon, but any identification determined to be updated with the output taxon matching the input (e.g. Species X) will not be touched.
Because doing a split this way won’t update as many identifications (e.g. updating identifications of Species X with identifications of Species X’), it will always be less disruptive. But it’s important to remember to manually narrow the input taxon by updating content such as the taxon range and common names that likely no longer match the narrowed meaning taxon after the split.