http://wiki.cubers.net/action/view/Server_hints_1.0.4 - The F*ing Manual Wrote:An AC server always uses 2 UDP ports: PORT & PORT+1. The default ports are 28763 and 28764. You need to use different ports, if you want to run several servers on the same machine (with one IP address). For example, use -f28765 for a second server.I don't blame you, though ;) The information is pretty hidden.
So, you'll have to use the command line parameter -fPORT with the appropriate ports.
EDIT: btw, that's still valid for 1.1