Ever wanted to start a new blog on WordPress.com and had to register a new account even though you already had one and just wanted another blog, not another login? That was pretty annoying, huh?
No longer! Now when you go to wordpress.com/signup as a logged in user, it lets you simply add another blog to your stable. As many as you want, need, or desire — no limits. You may also notice that when you're logged in the WordPress.com homepage gives you a link to the admin section of all blogs that you have access to.
You can now manage all the blogs you want with a single login and user for all of them. Enjoy. ๐ (And sorry for the old behaviour!)
Update: If you have multiple accounts, I would recommend going to Users > Authors & Users > Add User From Community to make one of them an admin on both blogs.