Open the Network Control Panel, beside configure there's a pulldown... pulldown "advanced" and there will be a port configuration box.
There should be one there for your current e-net card. Choose "new" and name the configuration. Choose the e-net card you want the IP to respond on.
Once you save, there is now on the 'configure' pulldown menu another option. Set it as you would any other address.
Voila.