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.