You can take advantage of exporting the whole jasperprint list:
List jpList = new ArrayList();
jpList.add(JRLoader.loadObjectFromFile("build/reports/Report1.jrprint"));
...
JRExporter exporter = new JRPdfExporter();
exporter.setParameter(JRPdfExporterParameter.JASPER_PRINT_LIST, jpList);
exporter.setParameter(JRPdfExporterParameter.OUTPUT_STREAM, stream);
exporter.exportReport();