using one MBPs screen as external display for another

You can't use one Laptop screen for the other except screen sharing but that is a "slow" software solution.

 

Apple's notebooks cannot be used as displays for any external video source. Sorry.

 

As others have said, you can't use the laptop's screen purely as a screen by direct connection. You can still use it as a secondary display, however, using a program like ScreenRecycler. ScreenRecycler has the other computer run a VNC session and then has your computer treat it as a secondary monitor. The video signals run through your network, so this isn't a good option if you have a slow network (although linking both systems by an ethernet cable might be a feasible option).

I used this program for a few years over an ethernet network at work with an old G5 and a large display. I didn't try playing videos through the other display (although the demo video indicates that it can be done with good performance), but it worked very nicely for me. The only unfortunate limit is that it only supports one other computer at this point - I had a few other systems and displays that I could have put into service otherwise