I just came across an immensely useful piece of software: ScreenRecycler (www.screenrecycler.com). It is basically a display driver mapping to a VNC server that allows your primary MacBook pro to provide the VNC server to another computer, and to use that VNC served display as secondary display on the first computer.
In other words, you install, then start the screen recycler software on your primary MacBook, and you start a VNC client on the secondary computer. This secondary computer can then serve as a secondary display to your primary computer, over the network, and you can configure and use the secondary display as if it was a locally attached display. You can manage it through the normal display settings on your primary computer, i.e. change resolution, etc.
This has just saved me like 1k USD for buying a second display just for the sake of being able to follow the usual running telephone conference and screen cast on one computer while putting the phone on mute and doing some productive work.
In terms of speed, it is not as if you had an actual display, since VNC is obviously slower than a display connection. Don’t expect to be able to follow videos on the second screen. But for the described purpose, it is ideal.