I am going to modify default magento category filter.

I have two categories Fruits and Apple. In fruits there are 100 products and in Apple there are 50 products.

www.demo.com/fruits.html (category page) this is listing all 100 products.

But now when i click on Apple from left category filter, it should load only 50 products that are assign to both category.

www.demo.com/fruuits.html?cat=10 (10 is Apple category id)

I am not getting how and where to modify code to get the result as per my need.

Thanks,