I have added a plugin to the dev section here : http://www.scirra.com/FORUM/topic58786_post362069.html#362069
<img src="https://dl.dropbox.com/u/1412774/iframePostMessageDemo/demo.png" border="0">
As you can see in the demo : https://dl.dropbox.com/u/1412774/iFramePostMessageDemo/index.html, you can exchange data between two iframes, each one containing a game made with C2. With that, you can do some 2 players "split screen" game !