Can anybody explain me why an xsd file has 2 sub files (.cs and .designer.cs) and what they are for?
As for the designer.cs, in our case it is generated by Xsd2Code. Why would you do that?
The assumption there is that the XSD represents a model you want to work with in your code - perhaps with XmlSerializer or as a DataTable. If you don't want that, wipe the "Custom Tool" setting for th xsd file.
The .designer.cs is usually the generated file; commonly the other .cs is for your partial class additions, but check before editing/removing.