While you're watching your buddy's live video, you can tap the new Request button that will show up in the comments area. You'll get a confirmation that your friend has accepted the request, and you'll get a quick moment to fix your hair and smile before getting connected. Then your screen will split in half and you'll be able to hang out live.
When you're doing your own live video, you'll be able to see requests from your own friends list, too. Simply tap the View button to add your buddy or cancel the request. There's a new little double smiley face icon, too, that will show you how many requests you have. Tap it to see the specific requests or to invite any viewers to join your live video. When you're done broadcasting, simply share the video to your Stories, or hit Discard to delete it completely.