if i understood you right, you can do this with an extra Network Adapter. Start Parallels (but do not start Windows)->Configuration Editor->'Add...' button->Network Adapter. So you will have 2 network adapters on your Windows which will let you to differentiate two networks.
What version are you using? If you are using Parallels Workstation 2.2 for Windows or for Linux, please, download latest update (build 2222). This version support up to 5 virtual network adapters.
For more information look at this announcement http://forum.parallels.com/showthread.php?p=95725#post95725