Can you make a Mac a second display to another?
This Frequently answered question (FAQ) comes about:
Can an iMac/iBook/Powerbook/MacBook act as a display for another Mac since they all have a built-in display?
The truth is complicated because you have to consider who is doing the processing, and who has the drivers for the work being done.
1. For people who want the processing power of multiple Macs together, there isDaugerresearch.com's Pooch.
2. Apple's Target Disk Mode lets you treat the hard disk of one Mac as an external drive to another. You do have to be careful with this, only the Mac that is currently booted to an active screen has its drivers enabled. If you need the drivers of the other Mac to do some sort of installation, it may not work right, or if you need it to run specific software.
3. The iMac 27" model began a new solution with its mini-Displayport, which later became a Thunderbolt port that allows displaying information from a second Mac. See Using a 27-inch iMac as an external display. There are some hurdles to get there, however some third party solutions for those hurdles are listed on:http://lowendmac.com/ed/bashur/09db/dvi-to-mini-displayport.html
4. If you have a second screen to initially setup a Mac as a server of some sort you can make it "headless" (i.e. no second screen necessary) using one of these solutions:
Microsoft Remote Desktop (pre-App Store versions), and App Store (Mac OS X 10.6.8 and later)
https://astropad.com/product/lunadisplay/ - is a third party option to make an iPad or Mac a second display of your existing Mac wirelessly.
The speed of your network though can make such solutions appear too slow for words. If you use IP over Firewire on Mac OS X 10.8, 10.7, 10.6 reasonable speeds can be obtained. This feature has been available since Mac OS X 10.3. Older links you'll find on the older tip are no longer valid.
Elgato makes Firewire compatible capture devices which can capture S-Video. If you output the video of that Mac to an S-Video connector, you can view its video on the Elgato capture screen in 720 x 480 resolution. This usually is too little real estate for an effective second display, but for some it may be the quickest solution available.
In addition, Mac Minis, Intel based iMacs and Mac Pros do accept any display that supports VGA or DVI. Contrary to what you might think, you aren't stuck to Apple's displays. The Apple displays themselves are true Progressive LCDs that are widescreen with color matching, extra ports, and an extra place to put an iSight mount if you can find an original iSight. DVI supports more resolutions, and dual-link DVI even more. Apple has supplemented info on the original iMac dual display mode on:
https://support.apple.com/guide/mac-help/use-an-imac-as-a-display-mh30822/mac
This is a revision of this tip to more current information: https://discussions.apple.com/thread/1587927?threadID=1587927
A related tip is:
Can AppleTV make my TV a wireless display of my Mac?