I'm not sure who has access to comm_joomla, but I can probably track it down. I searched through some emails, and the decision to use just @joomla was made to keep things simple for the public to follow the project via one account.
That would be really nice!
In my mind, I could be convinced either way, but it seems to me some people who setup the Twitter integration with the community.joomla.org site already considered the option of using a separate account and choose to use the main one instead.
Now that we have an active team for the Magazine, that might help to energize the Joomla! Community.
If we had two, for example, I can see a duplication of posts. For example, security updates, which account handles them? What about blog posts? Events? etc etc
Anything dealing with security issues, updates, or any core Joomla! issue will be on the main.
Community, Magazine, blogs, and events are all community related issues, so they would go out on the community twitter feed.
I'm sure it wouldn't very difficult to gather a list of what subject matters/topics will go in each twitter feed. They should be pretty obvious!
Joomla! Community Magazine Team Member
Follow Me at https://twitter.com/OnTheMarkDesign