A new feature that would allow two accounts to co-author a single message has been confirmed. A researcher has been posting evidence of the feature's development since at least December, but more recently discovered a dialog box about how the feature could work.
A co-author feature is something that would not be a problem for the micro-messaging service. There are a lot of reasons why two accounts might want to have their names attached to a single post.
You will only be able to invite public accounts who follow you to co-author a tweet if they accept your invitation. Hopefully there is a way to stop popular accounts from being hit with requests to co-author spammy accounts. The followers from both accounts will be shown to the top left of the post when the collaborative tweets are shared.
There is no word on when or if collaborative tweets will make it out of the development stage. At least one of the other competitors has such a feature.