I think the issue is the use of ports. Older versions of Umbraco were a bit flaky using a port as part of the domain. If you use a host name instead, that should work fine. As you're working on localhost, you could set up a hosts file entry for this for something like site1.local and site2.local, and have your umbraco site pick those up instead.
I believe it is possible to specify ports on newer versions of Umbraco, starting with when the pipeline was rewritten. If you used a more up to date version (at least version 6), you should be able to use ports in the manner you want to without issues.