How to filter posts on basis of more than one tag or category

Hi all,

I am facing problem while displaying posts from two particular categories.
Like for example, I have one classes menu that have classes from 6th to 12th. Now when one clicks on 6th class there I want to show a second sidebar navigation bar that have subjects like english, maths, sceince.

Now I have the second navigation as categories, so when one clicks on 6th then on english it shows all the posts under english category but I want it to show all the posts under both english and class 6th category.

Any help will be appreciated.

Please tell me whether there is a plugin available for this functionality or do I have to change the core functions for it.

Regards
Preity