First, this might be overkill:
rightclick on the worksheet tab that should have this behavior and select view
code.
Paste this in:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Cells.Count > 1 Then Exit Sub
If Intersect(Target, Me.Range("A1,C1")) Is Nothing Then Exit Sub
On Error GoTo errHandler:
Application.EnableEvents = False
Select Case Target.Address(0, 0)
Case Is = "A1"
If IsEmpty(Me.Range("c1")) Then
Me.Range("b1").Value = "Add PMI"
Else
Me.Range("b1").ClearContents
End If
Case Is = "C1"
If IsEmpty(Me.Range("c1")) = False Then
Me.Range("B1").ClearContents
End If
End Select
errHandler:
Application.EnableEvents = True
End Sub
You may not want the .clearcontents stuff.