Apple - How can I mirror one Mac's screen to another?

You can install the program "AirServer" on the Mac Mini to be able to AirPlay the display directly to it:

http://www.airserver.com/Download

Use Airdisplay by Avatron this way you can use your 2nd's Mac Screen just as a normal 2nd Screen attached to you first Mac.

Then you just need to set the mirrored option in System Preferences.

And as a Plus there is also an App for iPad so if you like to take your screen with you just do the same thing with you iPad.

An other solution is to use VNC which is built in on OS X and free (read more about it on http://www.davidtheexpert.com/post.php?id=5) You just need to enable the VNC Server (you do this in the System Preferences under -> Network screen shot follows when im @home on my mac) on the one Mac you like to mirror and on the other start the VNC Client (called ScreenSharing) and connect to your other Mac.