There are two decent programs you can use that are fairly easy to instal.
WarFTP is the one I use. Adding users for the first time and getting up and running can be frustrating...but after you figure it out, it's a decent little program.
Serv-U is very easy to instal and set up, but it's shareware and it's only free for 30 days or something like that.
Obviously, one of you will run the server, and the other will just use their FTP client to log in.
If you try WarFTP, I might be able to help you set it up. Basically, you select the little user (folder with little heads) and add your new user, then grant them FTP access. Then, you have to save the user database (in top menu options). If you're on a router, you'll have to open a port, in case you didn't think about that.