Multiple Selections in a Drop Down List in Excel - separator

Hi all,

I have a question here I was hoping to get some help with.

I have this VBA code to allow users to select multiple entries from a drop down list

It is working perfectly and furthermore it is working in 2 different columns thanks to these two lines:

Select Case Target.Column
Case 15, 16

Now, the separator for the multiple selections is currently a comma:

Target.Value = Oldvalue & " + " & Newvalue

Is there any way to have a separator for column O (case 15) and another, different separator for column P (case 16)

Full code here:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim Oldvalue As String
Dim Newvalue As String
Application.EnableEvents = True
On Error GoTo Exitsub
Select Case Target.Column
Case 15, 16
  If Target.SpecialCells(xlCellTypeAllValidation) Is Nothing Then
    GoTo Exitsub
  Else: If Target.Value = "" Then GoTo Exitsub Else
    Application.EnableEvents = False
    Newvalue = Target.Value
    Application.Undo
    Oldvalue = Target.Value
      If Oldvalue = "" Then
        Target.Value = Newvalue
      Else
        If InStr(1, Oldvalue, Newvalue) = 0 Then
            Target.Value = Oldvalue & " + " & Newvalue
      Else:
        Target.Value = Oldvalue
      End If
    End If
  End If
Case Else:
End Select
Application.EnableEvents = True
Exitsub:
Application.EnableEvents = True
End Sub