How to extend multiple controls of form in one extended class

Example I have a form with two controls one is button another one  is combobox.

Now I want to extend both controls with one extension class.

I have written below code but throwing an error.

[Extensionof(Formcontrolstr(FormName, Button))]

Extensionof(Formcontrolstr(FormName, ComboBox))]

Final Class Class_Extension

{

……………...

}

But the above code is throwing error like  extension of form is duplicated.

Please suggest if  anybody have an Idea to extend multiple controls at once.

Thanks