Joining two domains

I was in someone else’s office one afternoon last week and I noticed that when their users went to log onto Win2K that they had three option for the domain on the logon screen. 1) the local computer 2) domain “a” 3) Domain “b” I couldn’t not figure out how they were able to add one computer with only one nic to 2 different domains. If any one knows how to do this and can point me in the right direction it would be helpfull.
Thank you