You can do this using a VBA macro that traps the
Worksheet.Change event. For example this code will keep the cells
D10 on the same worksheet in sync:
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
Application.EnableEvents = False
If Target.Address = "$B$2" Then
Range("$D$10").Value = Target.Value
ElseIf Target.Address = "$D$10" Then
Range("$B$2").Value = Target.Value
Application.EnableEvents = True
In the VBA editor this code has to be stored in the worksheet on which you want to detect the change, not in the workbook or in a module. If you need the two cells to be on different sheets you'll need two macros, one for each sheet.
Note that we need to disable events before our macro changes the linked cell, otherwise that change will trigger another event and Excel will get stuck in an endless loop. Be aware that if your code exits with an error while events are disabled, you'll need to run a macro manually to re-enable them.