This was my first go at a Hackathon, so I decided to join a couple of peers from my class and set out to test the waters. We didn't go with a plan, since we were not quite sure how it would turn out, and basically we had no idea what it would be like.
Once we got there we sat down and started to jot some ideas. In the end we decided to do something along a split screen interface, but since doing a complete split screen would have been to lengthy, difficult and generally not a pleasant experience we settled for doing a split browser, in which one screen controls the position of the other and extends its canvas.
As can be seen, the second phone (Nexus 5, left, Rx) extends the screen of the primary phone (Nexus 4, right, Tx). Not only that, but it also controls the movement of the Rx through bluetooth.
The whole thing consisted in two different apps for each Rx and Tx. The communication was through plain text bluetooth, emitting the position of the top-left corner of the screen of the Tx, and from there adding some padding so that the screens match up.
In general, the app was quite buggy. It tended to crash a lot, and there were some errors in the bluetooth communication. Overall, not an application worthy of release. Nonetheless it was great fun and great practice as a novice Android developer.
You can learn more at the project's Challengepost.