2nd wordpress within same installation

Hello everyone. My desire is pretty simple. I currently have a wp install here ( http://www.leagueoflegends.cc ). It is in English. A friend wants to make essentially the same site (but with his own content) in German. I will create http://www.leagueoflegends.cc/de . I’d like to use the same theme across both. I’m no WP expert, but it seems to me that there are two ways to accomplish this:

a) create a new wordpress and install it in /de . the issue here is that it’s another DB, more maintenance, and I don’t know how to share 1 theme across 2 wp installs in different directories. I don’t want to have to change the files of two themes each time I make a change.

b) we both use the same wp install and his posts somehow get ‘routed’ to the /de page. his /de would use the same theme. this seems simpler, and I hope there’s a plugin that allows two wp’s on the same install.

thanks, and if you need me to explain further, I can.