two different theme/sidebar shown in two folder for one blog

I would like to show two different themes on my site. I would like to the public to see only one theme on the http://www.nameofmywebsite.com

I would like to be able to see a different them on http://subdomain.nameofmywebsite.com The blog would be install in the subdomain.

The sidebar I would like to show on the main site have less link on it. The sidebar I would like to show on my blog have more link on it. how do I set it up this way?