There isn't any easy config option to click or something to make that work. You would need to do it like an "external script" and grab the posts from the other blog. And I could imagine there is a way to make them all sort chronologically, etc mixed in with the first blogs posts on the main page, but that might be a little hard to do without a plugin - it would be a lot of smarty code in the template.
If you didn't mind having the posts from the other blog as a separate category of sorts, and they would only show up when you clicked on that category (as opposed to being mixed in on the main page) that wouldn't be too hard to do.
Though the posts from the second blog wouldn't show up in the rss feed of the first blog nor the search engine of the first blog, so maybe that isn't really what you want. Actually, I don't think plugins have any access to the RSS feeds (though they might - I've never tried) so it might be hard to get the search or RSS feed to work any way.
You could use the RSS plugin to import the second blog posts into the first one - I am not sure how that plugin works - I think it is usually used for a sidebar to show headlines of another blog.