Jeff Schertz has a great discussion about what ports to use when deploying the CWA server role in this post: http://blogs.pointbridge.com/Blogs/schertz_jeff/Pages/Post.aspx?_ID=80

Here’s a summary

During the deployment of an OCS Communicator Web Access Server there is a setting that is not covered in much detail in the documentation: the Communication Server Listening Port.  No default or suggested value is given. This port is used by the Communicator Web Access Server to listen for inbound communications from other OCS servers.  When an additional Virtual Web Server is added to the same host, as is common when both Internal and External types are setup on the same server, the new virtual site’s listening port must be on a different port then what the initial site is configured for. The post goes on to discuss how to alter the ports. Head over to see the full post.

There is no requirement on what port is used, except that it can’t already be in use on the host server. Therefore an idea might be to use 6051 and 6052 as suggested in a comment in the post.