Can I run multiple instances of screen-scraper on the same machine?
Yes, and you won't be in violation of your licensing agreement, either.
To do so you'll need to have separate copies of screen-scraper installed (copying an already installed instance works, too). You'll also need to add and modify a few settings to screen-scraper's "resource/conf/screen-scraper.properties" file so that the port bindings for the various instances don't conflict with each other. You'll also want to be sure that screen-scraper is completely closed before copying any files or editing any properties files. Here are the properties you'll need to add and/or change in the screen-scraper.properties file, along with sample values:
#Change to match the new install directory
InstallDirectory=C:\Program Files\screen-scraper professional edition 2\
#Change or add these properties
ServerPort=8780
ProxyPort=8779
DatabasePort=9852
SOAPPort=8458
WebServerShutdownPort=8551
The ServerPort and ProxyPort settings should already be in your properties file. The rest will likely need to be added. Just be sure that you select different numbers for each of the ports across the various instances you have installed. Also note that you'll want to alter the properties file only when screen-scraper is not running.
Once you've done all of the above, on Unix-based operating systems (e.g., Linux and Mac OS X) you can start and stop the server as you normally do with the "server" executable file.
On Windows screen-scraper installs as an NT service. If you install multiple copies on the same machine, each service needs a unique name. As of version 5.0, the installer has an option to specify the service name, so make it something identifiable to you. If you use the default name on all installations, only the last instance to be installed will run as a service; the others will need to be started via a DOS prompt in interactive mode. The process to do that is described in this FAQ.
Regarding licensing issues for this type of setup, you're free to use the same license on each of the instances running on the same machine. However, you would need separate licenses for instances running across multiple machines (one per machine).