These links might help:
http://en.support.wordpress.com/moving-a-blog/ (scroll down to 'transfer blog')
alternatively, if you just want to be able to log in to one account from the other, make yourself administrator of the other account for both:
Another alternative, if you don't have much or anything yet, in one of the accounts, is to create a blog with a similar (or different) title on the user account that has more blog content, then make the one you're not using on the other account, private so nobody else can see it.
Each account has one email address associated with it, but you can have any number of blogs on each account. Also bear in mind that while you can delete a blog, you can never delete a WordPress.com account.