Make use of the third possible State (vbGrayed) for the Value property and disable chkExtension when chkSpecialEvent is checked.
Try this code to see if it gives you the functionality you want.
VB Code:
Private Sub chkExtension_Click()
If chkExtension.Value = vbChecked Then
chkSpecialEvent.Value = vbUnchecked
End If
End Sub
Private Sub chkSpecialEvent_Click()
If chkSpecialEvent.Value = vbChecked Then
chkExtension.Value = vbGrayed
chkExtension.Enabled = False
Else
chkExtension.Value = vbUnchecked
chkExtension.Enabled = True
End If
End Sub
If you don't want to disable chkExtension use this code.
VB Code:
Private Sub chkExtension_Click()
If chkExtension.Value = vbChecked Then
chkSpecialEvent.Value = vbUnchecked
ElseIf chkSpecialEvent.Value = vbChecked Then
chkExtension.Value = vbGrayed
End If
End Sub
Private Sub chkSpecialEvent_Click()
If chkSpecialEvent.Value = vbChecked Then
chkExtension.Value = vbGrayed
Else
chkExtension.Value = vbUnchecked
End If
End Sub