We do exactly what you are trying to do - run the same stream (different bit rates) from the same server to the public on port 80.
The way you can accomplish this is as follows;
Option 1 - multiple IP addresses
On your NIC card bind two public IP addresses to your card;
For example; 184.108.40.206 & 220.127.116.11
Stream each of the stations on the seperate IP address bound to port 80 and your listeners can then listen to either stream on port 80 with no problems.
Option 2 - Proxy
Put a proxy server or firewall in front of your server - so that both the streams point to port 80 on the proxy server but use different URLS such as;
On the proxy server you redirect those requests to your shoutcast server with the appropriate stream port such as.
strm1.proxyserver.com ==> strm.shoucastServer.com:8000
strm2proxyserver.com ==> strm.shoucastServer.com:8002
It's important to understand that you are only redireting the request to the shoutcast server and getting back the response at the proxy server. The end user has no idea that the strm.shoutcastserver.com even exists - they only connect to the proxy server.
This is a more advanced method and requires more technical knowledge but is utterly flexible.
If you really only want to stream two or three streams, go with option one as it's less of a headache.