It's easy if you always update one cell and want the other to change, Vijay's
answer does this for you. If you want to be able to change either cell and
have the other update it's trickier.
You'd need to use VB and have a worksheet change event in the sheet. Press
Alt + F11 to get into the VB editor - find the sheet with the cells on it in
the project window & right-click the sheet then choose view code. Paste this
into the code window:
Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
If Not Intersect(Target, Range("A1")) Is Nothing Then
Range("B1").Value = Range("a1").Value
End If
If Not Intersect(Target, Range("B1")) Is Nothing Then
Range("A1").Value = Range("B1").Value
End If
Application.EnableEvents = True
End Sub
If your linked cells are in two different sheets then the principal is the
same but you need to have a worksheet_change event in each sheet and split
the above into two.