Try this.

Create SpinButton1 from the Control Toobox and set the Linked Cell
property to A1
and it's code as follows:

Private Sub SpinButton1_Change()
Range("A2") = 100 - Range("A1")
End Sub

Create SpinButton2 from the Control Toobox and set the Linked Cell
property to A2
and it's code as follows:

Private Sub SpinButton2_Change()
Range("A1") = 100 - Range("A2")
End Sub