Ola, your concept of "subset" and "parent" are off a bit. XML Schemas are not object-oriented and there is no concept of "subset" or "parent." During execution, there is only one XML schema and one XML document. The XML schema can be composed of many files using include and import statements, but conceptually there is only one XML Schema.
So, it sounds like you have created a customized XML schema based on the one at landxml.org. Your customized schema
must not include any design changes that conflict with the XML schema at landxml.org if you want your XML documents to validate successfully with both XML Schemas. If your design changes conflict with the XML schema at landxml.org, then there is no way to have your XML documents validate successfully with both of them.
You have to analyze the reasons why your have created a customized XML schema and you have to analyze why you need to be able to validate against the landxml.org XML schema.
And I think if I could somehow validate my manually created schema against the parent one, I will not have to validate XML files against both schemas?
Since your XML documents will contain markup that is not defined in the landxml.org XML Schema, you will never be able to validate your document with the landxml.org XML Schema. This is not possible.
XML Schemas are used to define an XML markup language. If you create your own XML Schema with additonal elements and attibutes and/or restructure something defined in a different XML schema, you essentially have created a new markup language.