One domain – two WordPress installations

Hi, I want to create two separate blogs on one domain. I understand how to use a single database with unique identifiers for each blog, but is it possible to share a lot of the includes between them? The most obvious being the stylesheet.

Apologies if this has been asked before, but looking through the amswers, they all seem to suggest putting the files you wish to include within the respective WordPress installation, and that’s not what I want. I want more of a “One Stylesheet to rule them all, One Stylesheet to find them, One Stylesheet to bring them all and in the darkness bind them” kind of a thing.