BuddyPress.org

@crashutah thanks you very much for your answer.

http://www.themainblog.com (the main blog with Theme 1 and the buddypress navigation bar in the header)
nameofablog.themainblog.com (the user’s blog url with Worpdress multi-bloggin with the wordpress theme of their choice)
http://www.themainblog.com/members/ (Theme 2, a theme made for buddypress like the one on testbp.net which will be used with the buddypress functions)

Or is it possible to have ?

http://www.themainblog.com (the main blog with Theme 1 and the buddypress navigation bar in the header)
nameofablog.themainblog.com (the user’s blog url with Worpdress multi-bloggin with the wordpress theme of their choice)
budypress.themainblog.com/members/ (Theme 2, a theme made for buddypress like the one on testbp.net which will be used with the buddypress functions)

Thanks you very much for helping me.

Best regards,