Co-tweeting is when you have more than one person monitor and respond within a single Twitter account. Recently there have been sightings that Twitter is starting to push a new co-tweeting feature that will identify the author of the Tweet. Twitter is now adding a by-line to the post. So instead of "5:52 PM Mar 3rd via web" you will see "5:52 PM Mar 3rd via web by jennadawn". Some say that this may be a premium feature that Twitter will be providing for their business accounts. By including this co-tweeting feature, business accounts will be able to have a more personal feel. Here is a example from the official @twitter account.