How to pivot a category

Today when I click on categories I get a url like http://www.mywebsite.com/?cat=6

But I all my categories hyperlinks to pivot around another category. e.g.

When I click category 6, results i want from category 6 and category 7.
http://www.mywebsite.com/?cat=6,7

When I click category 2, results i want from category 6 and category 7.
http://www.mywebsite.com/?cat=2,7

How do i achieve this? how to pivot a category?

I would like to have ability to change the value of pivot category.