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

Render each report and collect its pages.
var reportMerge = new Stimulsoft.Report.StiReport();
reportMerge.reportUnit = report1.reportUnit;

await reportMerge.renderAsync2();
reportMerge.renderedPages.clear();

await report1.renderAsync2();
report1.renderedPages.list.forEach(function (page) {
    reportMerge.renderedPages.add(page);
    page.report = reportMerge;
});

await report2.renderAsync2();
report2.renderedPages.list.forEach(function (page) {
    reportMerge.renderedPages.add(page);
    page.report = reportMerge;
});

viewer.report = reportMerge;

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