Dropdown category select – from two categories

Hello, I have looked around and didnt find any similar questions. I would like to get posts from two categories with help of two dropdown menus (select) and they dont necessarily have to be chained.

I would like to do this without using a plugin so I found this in codex: Dropdown_without_a_Submit_Button_using_JavaScript

Could someone help me to alter this code to get two select-dropdowns so that category.php gets the post from two selected categories.

Also you might notice that the example from above triggers the search without any button, in my case the search have to be triggered when you have chosen an option from the second dropdown (not first) is it posible?

Thanks for the help!