Red_Beret,
Fiz um pequeno teste e deu certo :
Botão no Form1
Private Sub Button5_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button5.Click
Me.Button5.Enabled = False
My.Forms.Form2.Show()
Me.Hide()
End Sub
Botão o From2
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
My.Forms.Form1.Button5.Enabled = True
My.Forms.Form1.Show()
Me.Close()
End Sub
Espero ter ajudado