Yes, but each instance of Web Crossing must be installed to its own unique directory and be given its own unique script name*.
Additionally, if you are running in Direct Web Service mode and each instance is serving content through the same port (typically 80), or you intend to use any or all of Web Crossing other features like email, NNTP, etc.,then each instance must have its own dedicated IP address (then after installation, bind each service via its control panel to the appropriate IP:port).
If one instance is for development and you run in Direct Web Service mode, give it a port number other than the one the live server runs on. If both instances are live servers, each must have it's own certificate. Contact our sales department at email@example.com for an additional certificate.
Of course one must take into account your system's resources: memory, disk space, existing load on the server in terms of traffic, available sockets/process, etc.
* the script name is chosen at installation time and is used to uniquely identify the service (NT) or process (UNIX). Initially this will mean that your site will be accessed using this script name as well, but you may change the "URL Script Name" in the Web Services Control panel after installation with no side effect on the service/process. Note: Recent revisions to the unix installer scripts do not ask you for the script name. You may change the default of webx after installation.