Standalone/On a Network?
Okay, I'm going to lay out what I think you're trying to do as I'm not positive with this.
You're running an XP system, that isn't joined to a domain or tree of any kind. However, you do have a current network connection attached to this system.
What you're trying to do, is to set this system up with an IP address that is external to the firewall so that you can test accessing the system from an external network (IE. Home, another office, etc...).
If this is the case there are several ways to test this out, but you'll want to move this disucssion to a networking forum, as all of them will involve setting up different network zones, router configuration, and some firewall tweaking.
The easiest way to do this is to take yourself off the network and use a DUN connection to the internet and test it from a truly external IP.
As for the two IP's, one for each NIC, it can be done. You can leave them dynamic to attempt to obtain their own address or you can static them. However, giving one an IP external to your current networking zone and an IP current to your internal network will not allow you to adequately test anything. Your system would act as a go between for itself and any programs that you wished to test.