The viewer and designer can share one dashboard, toggling between preview and editing by moving the report object between them.

Create both, then swap the report on toggle.
var designer = new Stimulsoft.Designer.StiDesigner(null, "StiDesigner", false);
var viewer = new Stimulsoft.Viewer.StiViewer(new Stimulsoft.Viewer.StiViewerOptions(), "StiViewer", false);

var report = Stimulsoft.Report.StiReport.createNewDashboard();
report.loadFile("../dashboard/Christmas.mrt");
viewer.report = report;

function openInDesigner() { designer.report = viewer.report; }   // show designer
function openInViewer()   { viewer.report = designer.report; }   // show viewer

How it works. The dashboard object is passed between viewer and designer, so edits made in one appear immediately when switching to the other.
На скриншоте ниже Вы можете увидеть результат выполнения данного кода:

Editing a Dashboard Template and Showing it in the Dashboard Viewer