Merchant Categories

I'll use the demo to help explain this one.

Click on Merchant to get a list of all the retailers listed. Click on Category to get a list of categories.

Is there any way I could merge the two so after you click on a merchant name, a list of product categories are show? Once you click on a category, products from just that merchant appear.

A practical example is if I clicked on Comet, I would be presented with a list of categories of products they sell (in this case, let's assume laptops and fridges). I click on Fridges and just fridges sold by comet appear.

Also, I use category mapping with some retailers. Would the above solution be compatible with mapped categories?

Regards,

Dave