Several rendered reports can be combined into one document by rendering each and adding its pages into a target report.

Render each report, collect its pages.
StiReport report1 = StiSerializeManager.deserializeReport(new File(path1));
report1.render();
StiReport report2 = StiSerializeManager.deserializeReport(new File(path2));
report2.render();

StiReport reportMerge = StiReport.newInstance();
reportMerge.getRenderedPages().clear();

for (StiPage page : report1.getRenderedPages()) reportMerge.getRenderedPages().add(page);
for (StiPage page : report2.getRenderedPages()) reportMerge.getRenderedPages().add(page);

reportMerge.setIsRendered(true);

How it works. The merged report holds the rendered pages of all sources, so multiple templates present as one continuous document.