The model registry currently only supports the main database connection. The Contao framework supports opening additional database connections, however, you cannot make models use them.
More information: contao/core#6248
Non-admin users cannot copy or move content elements between different parent types, e.g. from an article to a news item or from a news item to an event. They can only copy or move elements from e.g. one article to another article.
More information: contao/core#5234