I am afraid that Tracker Software will not make modifications anymore to their PDF-XChange Viewer that has become obsolete since a while.
I suppose you know that PDF-XChange Viewer has been replaced by PDF-XChange Editor:https://www.tracker-software.com/produc ... nge-editor
In the Editor, you can also open a new window for each open document.
Even though, in Windows Task Manager, you will find only one instance of PDFXEdit.exe vs multiple instances of PDFXCview.exe
If you have a valid license for the Viewer Pro, then it should also be valid for the Editor.
If Tracker Software Support is reading with us, they can maybe confirm this?
Sorry if I should be wrong in my license interpretation.