Sounds to me like it's formally just one shared residential unit, because the separateness is missing. So not easily separable if the parents would like that as a separate property or something. And if you ever renovate something where there might be funding per residential unit, that probably wouldn't count as a separate unit either.
If you want a proper two-family house with separate apartment access, you should take a closer look at what the development plan says about that. (In our case, it explicitly states that only single-family houses are allowed in certain areas or a second residential unit only if it is subordinate, or something like that. I don't remember the exact wording anymore)