This cannot be done.
The classical desktop and the modern apps are two separate worlds.
- Classical desktop applications are those that (can) run on previous versions of Windows.
- Modern apps are the new thing in Windows 8, they didn't exist before. These applications have new API to the system and are run in a more restrictive environment than regular (or classic) desktop applications.
The only exception is made to browsers which can use the full Windows API even in modern interface, and can be run as either desktop or modern mode.
You cannot start multiple desktop "apps" because it's not really an application, it's just the "legacy" Windows desktop presented as a tile.