Can't import an XML file AND use schema for validation

Question

I'm new to XML. I want to be able to import an XML file from a main one. Additionally, I want to be able to validate the combined XML using a schema.
Per this, I was able to import an XML file into another. Per this (but independently) I was able to use my schema to validate my XML. Problem is, I am struggling to get both to play together. I have a lot of technical details from my struggle to get this to work, but for now I'm thinking none of that may even be necessary if it's already known that this is simply a limitation of XML. The problem seems to stem from fact that if a DOCTYPE is provided, it is used for validation in place of a schema file, yet DOCTYPE is needed to import another xml file. The main thing I've been experimenting with is the DtdProcessing property of the XmlReaderSettings used for creating the XmlReader.