Search posts with multiple categories

I have 2 cateogries with ID: 10 and 16.
I need to get all posts that included in both in 10 and 16.
- If I use http://....com/?cat=10,16 , it shows all posts which in 10 or 16 category.
- And I use http://....com/?cat=10+16 , it shows like that too.

I only want to show all post in both 10 and 16,but http://....com/?cat=10+16 ,it's not working.

How can I solve it ?Help me plz .Thanks