There are three general scopes in Magento:
This where you can have different websites (or businesses), like awesome.com and horrible.com.
Products from one website do not mix (are not shared) in the same cart as products from another site.
This is where you can have sub-brands or multiple brands under one roof, for example: houses.awesome.com, cars.awesome.com.
Technically you can probably have both awesome.com and horrible.com point to different stores under the same
website, but it would probably defeat the whole purpose.
All products from each
store are shared in one shopping cart (but not all products will be 'editable' in cart)
This is where the same domain links to multiple variations, for example: having and english version of awesome.com, as well as a spanish version of awesome.com.
store views can be used: different customers get different experiences.
All products mix and are editable in one shopping cart.
It sounds like you need to use multiple
stores under one website to achieve your result.
For more info on this subject see here and here.