Adding laptop to 2 domains, How?

How do you allow a user in Windows 2000 to choose which domain to connect to when the user works in two totally seprate busineses with two totally seprate networks/domains. At the moment, when the user is working with us, he has to ring one of our IT guys to come down and connect him to our domain in the usual mannor. When he goes to the second site, they do the same and overwrite our settings? Does anyone know a way around this in win 2000??? Any suggestions?