There are multiple threads about this issue. A good summary of the actual situation was posted by Stephen Mack: https://www.odoo.com/forum/help-1/question/how-to-setup-multiple-websites-in-odoo-v8-85509

Hello Sr. Yuri. I need a multi company and a multi website.

For example:                     

Actually i configure a multicompnay and create 2 website. Configure nginx to use both domain, how can i define a different theme for each website.

Thanks very much.

Best Regards. 

I know that multi website is supposed to be in V9, but didn't know if there was anything yet.

I was looking for an answer like that for a while, finally I start doing my own because there is any thing like that. You can start by using product_brand and website_product_brand that you'll find on github. With little modifications you can build relations between brands and companies, and make possible this diferent ecommerce.

Write me if you wish