Twitter is testing a new feature of switching accounts in the process of replying. Through this users will be able to manage multiple accounts in one go. There’s no official statement from the company regarding the same testing.
Exact similar options are already been available in the TweetDeck where users can switch to a different account at any point in time while responding to a tweet. Many of the Tweetdeck features are now being implemented in the main app which is good in a way.
Image Source: Social Media Today
This feature is available to some users for a limited period of time and for some users it’s not even shown. There are no such parameters of who can avail of this and who doesn’t.