Although there is an official Twitter Notify plugin from Microsoft for Windows Live Writer, it can be used to post to only one account at a time. Live Tweet is a simple plugin for Windows Live Writer which enables users to post to three Twitter accounts when you publish a post. You can also customize the tweet by adding keywords and other details. You can download the plugin from here and once installed, naviagte to the plugin options.
You can configure the accounts using the option page for the plugin. You can specify upto three Twitter accounts in the options. Once you click save, they are validated. There is option to customize the Tweet using your message and keywords.
Once configured, next time you publish a post, you get a notification of tweet where you can select the accounts to tweet including customizing the message.
Live Tweet is a simple and yet useful plugin if you have multiple twitter accounts.