Tag Drop Down Form

Hey! You may have seen some of my posts regarding the ability to sort posts by their residence in two distinct categories. It doesn’t appear to be feasible; however, tags are perfect.
https://www.blog.com/?tag=tag_a+tag_b does exactly what I want.

Now the question is, can someone help me make a drop down menu that will function similarly to the category dropdown widget? I actually need to make two menus. One that onchange submits the first tag to a variable and the second that onchange submits the url/?tag=menu1var+menu2.

Thanks!