Is it possible to use two different themes for BuddyPress and WordPress ? If yes, please tell me how.
WP - 3.8.1 BP- 1.9.2
In general - no. BuddyPress tightly integrated into WordPress, and uses it's templates, just adding some own html pieces on pages.
Of course, using some hacks you can change that, but that will result to problems in UX and maintaining all that code.
I think it's much better to have 1 theme, but for BuddyPress pages you can add additional styles and change html - BuddyPress allows to do that, you can redefine any template file.