Install different themes for Wordpress and Buddypress under one site

I am running Genesis by Studiopress using the Church child theme.

However, I want to install Magazine theme for Buddypress whilst still having the Church child theme, under Genesis, on my main site.

I cannot find anywhere on WP DEV how to do this. A video tutorial would be FANTASTIC!

I have enabled Buddypress on the site. It is all screwed up! All the Buddypress content appears UNDER the main site.
http://www.musicwise.com.au/

I have not activated the Magazine theme yet as the buddy[ress code is all screwed up - scared it may stuff up all the work I have TRIED to do thus far and take over the Church theme.

Not having much luck with plugins - but I will plug away at it (lame joke - but did it bring a smile?).

I am in Melbourne, Australia. If anyone is on this timeline (7.40pm now) I would appreciate the help.

I can't add any screenshots through any browser - IE, Chrome or Firefox.

Cheers
Judie