"Where are you from, Iain and Andrew?"
Translation:Cò às a tha sibh, Iain agus Anndra?
sibh is both the polite and the plural form.
thu is the informal form used with one person you know well.
So, if you are speaking to one older/revered professor or grandparent, you would use sibh. And you would also use sibh if you were talking to two or more people you know well and speaking to them as a group.
"Caite a bheil sibh a' fuireach?" means "Where are you living?", which is asking about where you are living just now. However, "Co as a tha sibh?" meaning "Where/who are you from?" is actually asking about your origins, your family, clan, or place of birth. . .literally as someone explained to me, "Who/Where are you out of?" Hope that helps. :)