BuddyPress.org

Hii there,I am new to buddypress.I have been using wordpress for a while.now i want to integrate buddypress with my native blog but i dont want my old theme to be changed.I want to use 2 different themes but at the same time I want users share same database that is users registers only once.

I dont have wordpress multi site installed.I can afford to install buddypress on subdomain.
here is my site http://www.ziddii.com

For example I have installed amazon affiliate plugin which works within my site,share same database but use different theme.here is the link.
http://www.ziddii.com/?aa-shopHome

Exactly this thing i want to do with wordpress.
Please tell me how to do it without enabling multisite

any help will be appreciated