Dorico can handle several independent pieces in one document.
Will the MusicXML-standard be enhanced, so that such documents can be exported in full?
PS. Steinbergs lates newsletter state, that other scoring programs can only handle a single piece in one document.
That is not enterely true. So far the only (other) scoring program, that handles more pieces in one document, is PriMus, to be found here: http://www.columbussoft.de/?lang=en
And for those who speak Danish: http://www.indigo2.dk/primus