How to test asynchronous website activity between two users

We're trying to write BDD tests using Jasmine, and a common use case we need to handle is:

User A: Does some action
User B: Gets notified after xx ms that the action has happened asynchronously (either via WebPush or polling)
User B: Responds to user A's action
User A: Receives B's response

I'm struggling to build an automated test for this scenario. Our current tool-of-choice is Jasmine, but we're flexible at this point. How can you handle multiple logins simultaneously, especially where there is some delay between responses?