Hello Davo,

Are these ComboBoxes on a worksheet or a VBA UserForm? I am guessing worksheet. Add the code below to a VBA module in your workbook.
Sub SelectCell(ByVal ComboBox_Number As Integer, ByVal KeyCode As Integer)
    If KeyCode = 13 Or KeyCode = 9 Or KeyCode = 39 Then
       Cells(ComboBox_Number + 4, "C").Activate
    End If
End Sub

Call the macro from each ComboBox_KeyUp event like this...
Private Sub ComboBox1_KeyUp(ByVal KeyCode As MSForms.ReturnInteger, ByVal Shift As Integer)
    Call SelectCell(1, KeyCode)
End Sub

Private Sub ComboBox2_KeyUp(ByVal KeyCode As MSForms.ReturnInteger, ByVal Shift As Integer)
    Call SelectCell(2, KeyCode)
End Sub