I've also not managed to get mutliple IP connections working in FS2NET. Instead of connection through FS2NET, you can set up your own local server and play there. Ofcourse you can't join internet servers that way and no-one outside your local network can join, but at least you and your friends can play simultaneously within your network.
You'll have to change a few settings to play on LAN.
Go into your /freespace/data/multi.cfg file and remove everything (back up the old file somewhere). Then add the following line:
Now save the file. In the FS2 Launcher, set the multiplayer connection to LAN and leave the Force IP and Force Port fields blank. You can enable "Allow in-game joining" in the Features/Experimental tab. Run Freespace 2.
In Freespace 2, create a multiplayer pilot. Then go into Options - Multi. Enable TCP, and /Multidata. Disable PXO. Do not set any IPs. Apply and commit the changes. Go back to lobby and click on Create Game and set up a new server. If you've allowed connection through your Firewall, you friend should be able to see your LAN server.
More info here: http://www.hard-light.net/forums/index.php?topic=81650.0