There is a schema language called NVDL supported by Oxygen which allows validating part of the XML elements with one schema and other elements from the same XML with another but it does not support DTDs, so if you had two XML Schemas for validating various parts of the XML, you could use NVDL for this, we have a sample in:
Could you maybe convert the DTD to XML Schema using our Tools->Generate/Convert Schema menu?
Otherwise maybe you could use our Java API ro.sync.exml.workspace.api.editor.WSEditor.addValidationProblemsFilter to add a custom filter which adds to the validation problems. This filter would allow you to maybe get from the main XML document those fragments which need to be validated with the DTD and validate them with Java code, then create validation problems for them and add them to the list.
<oXygen/> XML Editor