|Excel cannot have a formula in a cell and allow for user input directly into the same cell. If there is a formula in a cell, then it would get over-written when user enters 60.|
There might be a way to use a Sheet_Change macro to do what you want, but you would need to be more specific.
For example, this would do what you asked. Right click the sheet tab, choose View code and paste this in the VBA window.
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" Then
If Target = 60 Then Range("A2") = 40
If Target.Address = "$A$2" Then
If Target = 60 Then Range("A1") = 40