Connecting a 2012 iMac and 2015 iMac for dual screen setup

Use iMac as a display with target display mode - Apple Support

You could not use the 2015 iMac as a Target Display for the 2012 iMac. Target Display Mode went away in Late 2014, when the first 27" 5K Retina iMac came out. No 2015 iMac supports being a Target Display.

You could use the 2012 iMac as a Target Display for a 2015 iMac – if you downgraded macOS on both machines. This would require clean reinstalls.

• The 2012 iMac would need to be running High Sierra or earlier.
• The 2015 iMac would need to be running Catalina or earlier. (Apple added restrictions such that no Mac which was released after 2019, or is running anything after Catalina, supports using any iMac as a Target Display).
• The cable connecting the two would need to be a Thunderbolt 1 or 2 cable. This would have the same physical connectors on each end as a Mini DisplayPort to Mini DisplayPort cable, but would be one constructed to carry traffic at Thunderbolt data rates. Such cables are no longer sold new, and may be difficult to find used.

Also note: If your goal was to run old DRM-encumbered one-time-purchase software (like old copies of Adobe Photoshop or Microsoft Office), you might find that you could not transfer your software to the 2015 iMac, due to the vendor having removed support for activating old software.

imac222 wrote:

hello, I have a 2012 27" iMac running catalina.. i would like to add an additional 27" iMac for a dual screen set up.. there is one for sale that is 2015 running big sur. can I hook these together with thunderbolt or some other way? thank you for your time.

I just did a quick search, and people were asking something like $65 – $80 for Thunderbolt 2 cables.

You can get new 27" 2560x1440 pixel monitors for less than $150, some of which even have decent (near-100%) coverage of sRGB. I'm not sure it makes a whole lot of sense to buy a used 2015 iMac (with the risks that entails), and force it to stay back on Catalina, and spend another $65 – $80 for a Thunderbolt 2 cable on top of that, to use your 2012 iMac as a less convenient substitute for a 27" 2560x1440 monitor …

… instead of just buying a 27" 2560x1440 pixel monitor and a Mini DisplayPort to DisplayPort cable to use with your 2012 iMac (which might cost less than the 27" 2015 iMac plus the Thunderbolt cable)

… or biting the bullet and upgrading to a current system (say, a Mac mini paired with a 27" 4K monitor)