Basic formula for linking data

I'm trying to figure out how to link data from cells into new cells. For example if cell A1 = Cell B1 then I want Cell A2 to equal B2, but if A1=C1 then I want A2=C2and so on. The purpose is to prevent the user of my sheet from having to enter two data items that are always linked. Can anyone help?
The cells being referenced are unchangable, so it should be possible....Thanks