How to manage a screencast with two Android devices?
If both devices are rooted, you can use AirDroid, which offers an...


  Realtime Android screen (experimental feature, root permission required)
  View your Android's screen, take a screenshot, or even record a screencast.


As you connect to the device using a web browser, you can just have two browser windows open - et voila, both devices should be shown simultaneously.

Alternativ solution: droid VNC server (and other VNC servers). Again, requires rooted device -- plus a VNC client on the computer.

There are several more solutions, but they mostly fall in either of the mentioned categories.