Twitter is allowing two accounts to jointly post a Tweet. The CoTweet feature allows a person to tag another person. The co-authored Tweet is posted with two profile pictures and usernames.
Co-authoring RSS
Today at its Build 2019 developer conference, Microsoft announced the Fluid Framework, aiming to "break down the barriers of the traditional document as we know it" for better collaboration.
Microsoft today announced a number of collaboration features such as co-authoring in Excel and new Yammer apps that are now available for Office 365 users as part of the August 2016 update.
Office 365 users on the Fast ring will now be able to use the co-authoring feature. This will allow your team to collaborate by seeing who is working on a document and what changes are being made.