You should really use Option Buttons. They do that withouth the code, but still:

VB Code:
  1. Private Sub Checkbox_Click(Index As Integer)
  2.     Select Case (Index)
  3.         Case 0:
  4.             If Checkbox(0).Value = vbChecked Then Checkbox(1).Value = vbUnchecked
  5.         Case 1:
  6.             If Checkbox(1).Value = vbChecked Then Checkbox(0).Value = vbUnchecked
  7.     End Select
  8. End Sub