Loggin on to different networks

I have been charged with setting up a Windows 2003 network, which in itself is not an issue. I have 15 workstations (2000 or XP) which currently log on to an entirely different network, but must be used to log onto the ‘new’ network. Neither the two networks should meet!! (don’t ask me why, I steer clear of politics!). My inital thought would be to add an additional network adaptor to each pc, and then the user will have the option to logon to whichever domain they wish. My problem is how do I go about this? Any suggestions gratefully recieved.