If you have for example, two different blogs using the same files for appearance, how would you make a certain category post appear for each blog?

I mean, I want 'post a' to be the only post you see when you first go to 'blog a', and then I want 'post b' to be the only post you see when you first go to 'blog b'. Then, if you want to see what else there is to read, you would simply click on the category links to view those posts.