It is not possible to run multiple instances of Scan2CAD on the same computer at the same time.
If you have seen a warning stating that Scan2CAD is already open, this is because you have Scan2CAD already running and performed an action which has attempted to open the application for a second time.
You might see this warning if you use the ‘Open With’ option on Windows to open a file in Scan2CAD, when you already have Scan2CAD running. This is because the ‘Open With’ option launches the application and opens the file. This would therefore create two instances of the software.