You need to pinvoke SetCurrentProcessExplicitAppUserModelID()
supplying the same AppID for all the applications that you want to share a taskbar button. The OS will then treat your 5 applications as if they were the same application.
Make sure to call SetCurrentProcessExplicitAppUserModelID() before any of your application's UI is displayed.
[DllImport("shell32.dll", SetLastError=true)]
static extern void SetCurrentProcessExplicitAppUserModelID(
[MarshalAs( UnmanagedType.LPWStr )] string AppID );
private static string AppID = "some guid"; // use the same ID in all 5 apps
...
SetCurrentProcessExplicitAppUserModelID(AppID);
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