The quick instructions on how to do this on Windows XP machines.
Check out this post and download this PDF. Read section one. Now you are almost in business.

Issues and questions you may have:
If you have been running your Skypes on the same profile and logging out of one and into the other when you begin running the second iteration you won't have any buddies. You will need to fix that by copying a file to your new windows profile. Instructions are here.

In this simple configuration you can't both be on Skype at the same time. If you answer a call for her while you are talking to someone else you have a partyline! Of course it is possible to chat on one profile and talk and chat on the other too. If you want to get more fancy then use a second sound card and treat the system just like the ipod radio above. If your PC is powerful and you have plenty of bandwidth one can talk in one room on a bluetooth headset while the other is typing and talking on the PC.

Have fun!