How to use screen of 2nd mac?

I have 2 MBP and would like to use the screen of the second one as an external monitor for the other one. I use an external monitor at work, but would like to know how I can use use my home MBP as a second monitor when I bring my work MBP home.

thnx
p.s. buying a monitor is not an option.

You can't do this...might be something in looking at simulation software but only the recent iMac's can do this.