What you want to do is have one basic stylesheet that contains the basics (Such as basic page layout and so forth) and then load new stylesheets to override the defaults whenever you need them.
Alternatively, if you know the page is probably going to have the other stylesheet used, you could perhaps simply change the class of the body and the second stylesheet would take over. However this would be less efficient and much more complicated (On the CSS level) because every file would need to add body.class before every block.
