Unfortunately, the way WordPress.com’s service works is by connecting the plugin (which the WP service views as “an app”) to a specific blog as its client. At the moment, this means the plugin can only cross-post to one destination blog, because it can only connect itself to one blog. As far as WP.com’s concerned, each of your blogs is its own separate account, even though they’re all associated with your account behind-the-scenes. It’s my hope that WordPress.com will change the way they limit apps’ access to your account for this reason, but I’m not sure when or if they will do this. ??
In the mean time, I understand from some users that you could use the self-hosted WordPress’s Network feature (formerly called “multisite”) to create multiple blogs with the same content that cross-post to different blogs on WordPress.com. As a disclaimer, please note that WP-Crosspost is beta software and has not been tested in this way.