Hi Louis,
To clarify on the overwrite - If you submit JSON via API, and then XML via file upload, then the XML will overwrite. But if the XML is also through API, then you have the choice to overwrite or append. For example, if you're using the API, you would POST /measurement-sets with the first set of measures (in JSON), and then use PATCH /measurement-sets with the second set of measures (in XML) to add it to the same measurment-set. So you'll need to use the same submission method (API or upload), but can use different different formatting (JSON vs XML).