two installations, same server

It's not only possible and very reasonable, but we do it every day here at Scout, running dozens of CWIS installations on the same server.

I think the only place you may possibly run into trouble is with links between the two installations, if URLs are being auto-generated and for some reason the application framework uses the wrong base URL.  (No specific situation comes to mind where this would occur, but since base URLs are derived and URLs generated using certain assumptions and Apache allows you to set up an amazing variety of directory arrangements, we definitely haven't tested every possible configuration.)

If you do run into any difficulty with this or have questions, please let us know and we'll be happy to try to help.