This a great tool if you wish to spam everyone at one time. Most people that have multiple Twitter accounts have them for very specific reasons or business cases. Such as the 2 that I have. But Mattinator, a UK company out of Bath, is apparently a few day project by a group of people (you can see their pic on the About page).
Reading the help files, you simply log in once with any of your Twitter accounts, then add the others. Simple and quick. The interface is clean and they have a great welcome video on the homepage.