Although I don't see any benefit in this approach:
Private Sub TextBox1_DblClick(ByVal Cancel As MSForms.ReturnBoolean)
UserForm1.Tag = 1
UserForm1.Show
End Sub
Private Sub TextBox2_DblClick(ByVal Cancel As MSForms.ReturnBoolean)
UserForm1.Tag = 2
UserForm1.Show
End Sub
Private Sub UserForm_Activate()
TextBox1 = Sheet1.OLEObjects("TextBox" & Tag).Object.Text
End Sub
Private Sub CommandButton2_Click()
Sheet1.OLEObjects("TextBox" & Tag).Object.Text = TextBox1
Hide
End Sub
Private Sub CommandButton1_Click()
Hide
End Sub