You can fudge it. :)
Figure out which of the two accounts you want as your main one. Go into the Dashboard of your second account and add in your main account as an Admin there. You can do this at Dashboard -> Users -> Authors & Users. Again, you have to assign them a level of Admin for the account to have complete control over the second blog.
Quick aside - You can learn about the different user levels here just for your reference.
As to moving, there is currently no offical method of backing up the data here currently although it is planned. The unoffical method is via using RSS. Here's a link directly to a Post explaining the method. Do note that RSS output changes things like quote and other marks into escaped characters. You're going to have to do a search and replace before importing.
Do remember of course that you need to set up WordPress on your own server space, you are responsable for your own updating and all that, and you can't take your WP.com domain name with you.
Hope this helps,