Licensing on "hot spare"

I have a setup with a hot spare machine which is used for development, but is primarily a backup that can be activated at any time should a major failure occur on the production machine. The same 14 domains are installed on both machines.

The backup machine is accessible via ip address only, the dns for all domains points to the production server, but the backup machine is fully configured and ready to become fully active at any time.

Considering that this hot spare machine is running all the time and connected to the Internet, will I need to purchase a separate license for Virtualmin Pro on this machine?

Regards,