Running multiple stores on one database

Can anyone help me with setting up ProductCart to have 

multiple niche web stores all run by one common inventory and billing system (running on one database)

An diagram of what I am talking about is located here:

http://www.zoovy.com/howitworks/multiple_online_stores.html

The setup described allows you to run multiple web stores, each with a completely different look and feel.

A common product and billing database contains the information for all of your stores in one place, allowing you to track all of your inventory and orders in real time, through a single interface.

My problem is that I sell skin care and cosmetics, and I want to take a couple of categories from my current Web site ant turn them into niche sites. You can view my site at:

www.fairlightcosmetics.com

I want to take the Natural & Organic Category, and create a site with just those products, so that those products in that particular category are the only products shown on that site.  The reason being is that people who want to buy Natural & Organic products do not want to shop from a site that also sells chemical-laden skin care and cosmetics, but those who shop from the main site with all the cosmetics listed will buy both traditional skin care & cosmetics as well as the Natural/Organic stuff.

Hence the need to separate out the Natural/Organic stuff with it's own site for those who are offended by seeing both types of cosmetics.

But, I do need to do the Admin stuff and track inventory from the same database because I have tried setting the Natural & Organic up in a separate shopping cart, and tracking the inventory is a nightmare, as well as processing orders from 2 carts.

I need link the niche site to the same ProductCart database as the regular store so that I can track inventory, and so that those wanting to order products from one of the other categories can still do so and have the same shopping cart to add the items to, and all have the same checkout.

I want a customer logging into the niche site to access their info stored at the main site and I want the shopping cart to follow the customer if they leave the niche site and want add items from the main site as well.

Again, I would need to have that niche category developed as a separate Web site, but linking to the same database, and having it's own look and feel, until it gets to the checkout section of course, where it would be directed to the main site for checkout.

From what I understand, the ProductCart license allows for running more than one niche site on the same database, but, after extensive research, I have not been able to figure out what I need to to to achieve this.

If anyone could give me explicit instructions, I would so much appreciate it - I know that it is probably a long and involved process to answer this, but I would really like to be able to set my sites up this way and if anyone can help, I would be so grateful.

Thanks so much!
Catsully7