Trying to run two demos side-by-side, one for 1.4.13 and one for 1.4.14,
I get on the one started later:

$ python demo.py 
Traceback (most recent call last):
  File "demo.py", line 179, in <module>
    sys.exit(main())
  File "demo.py", line 175, in main
    run_demo(home)
  File "demo.py", line 124, in run_demo
    roundup_server.run(success_message=success_message)
  File
"/path/to/roundup/roundup-1.4.14/roundup/scripts/roundup_server.py",
line 923, in run
    httpd = config.get_server()
  File
"/path/to/roundup/roundup-1.4.14/roundup/scripts/roundup_server.py",
line 646, in get_server
    % self["PORT"]
socket.error: Unable to bind to port 8917, port already in use.

If they're sharing the same instance then yes, they won't try to use a separate port. The conflict 
check is only done when the demo instance is created. I'm not particularly interested in making 
it more sensible for your specific use-case, sorry.