A Windows workstation can only belong to one domain at any given time. If you need to authenticate to two different domains that do not have a trust relationship established, you'll need to choose one to which to authenticate, then provide separate authentication whenever you access any resources on the "untrusted" domain. If it's more a question of switching back and forth between two different domains, I've had a lot of luck with Multi Network Manager, a product from Globesoft. It not only allows you to set up different IP addresses (and the associated gateway & subnet mask information), it also allows you to specify different domains, network shares, printers, etc. I found it very useful and easy for my end users to take advantage of.
This was first published in April 2003