Instagram began testing combines live broadcasts, stated in the official company blog.
To add another user to your broadcast, simply click on the icon in the lower right corner (it is in the form of emoticons) and select the user from the list. You can connect any user who is already watching the stream. At any time you can delete a user from the broadcast and replaced by another. In addition, invitees can leave live independently.
During a joint broadcast screen is divided in half. Viewers will still be able to leave likes and comments. A new feature available to a limited number of users, but after a few months it will be available to everyone.