Sorry that you had some confusion about this.
It seems like theres a couple concepts we should clarify here. There should only be ONE session for both your users. Calling them 'sender' and 'receiver' separately doesn't make sense if you intend to set up a bi-directional conversation as you described. Instead, what we need to understand is the concepts of 'publish' and 'subscribe'. In fact, both users will be 'publishing' which means they will send audio/video data that originates from their own computer. Also, both users will be 'subscribing' to exactly one stream – the stream that is coming from the other user, which means they will be displaying it on their own screen.
Now given all this, setting up a bi-directional conversation is as simple as having each user (1) connect the the (single) session, (2) publish their own stream, and (3) subscribe to any stream that is not their own. BOTH sides are running the exact same code. The only thing left is to make sure you give that single session ID (and corresponding token) to exactly only the two users that are trying to have a chat, but logic will probably be specific to your application and done on the server-side.
Okay, now that we have all the concepts nailed, here is what you should try to do:
1) Try out the Basic Tutorial (http://www.tokbox.com/opentok/api/tools ... orial.html
) using your own API Key, Session ID, and Token. Open this in two separate browser windows on your computer and you should have this bi-directional chat working.
3) Add your server-side logic of getting these two users to each other on top of what you already have.
I hope that helps,