Although this topic is quite old already, BlueKnight, it's possible to play with 2 accounts on 1 IP on the crpg servers.
The problem here must be connected with the cd keys.
@ OP: Does either of you use a steam account? Whenever you changed the cd key in the regedit and start warband by steam, the steam cd key gets selected and changed in the regedit. Therefore, if you want to play without the steam cd key but with the other key on one pc, you got to install the game again (download it from taleworlds.com, and change cd key then and select "do not start with steam" when the crpg loader asks you).