Hi Oliver,
You need first to remove the NUIDocument from DocumentManager.Documents using RemoveDocument and then add it in the desired position:
m_Manager.DocumentManager.RemoveDocument(doc1);
NDocumentHost host = m_Manager.DocumentManager.ActiveGroup;
m_Manager.DocumentManager.AddDocument(doc1, host, desiredIndex);
Of course, it applies when document view is MdiTabbed.
Best Regards,
Nevron Support Team