Using 2 iMac screens

I could be wrong, but it looks like the iMac in the first picture has Thunderbolt 1 or 2 ports with Mini DIspayPort connectors (lightning bolt symbol), while the iMac in the second picture has a Mini DisplayPort (rectangle with two lines on the sides, suggesting a screen).

• An iMac with a Thunderbolt 1 or 2 port might be able to use an older iMac with a Mini DisplayPort as a second (TDM) display. Without the information from About This Mac, I can't say whether your particular iMacs would qualify.
• An iMac with a Mini DisplayPort cannot use a newer iMac with a Thunderbolt 1 or 2 port as a TDM display. The older iMac is not capable of generating the Thunderbolt signal that the newer Mac would require – if the newer iMac supported TDM at all.

So the answer is "No" in one direction, and "It depends" in the other.

On each iMac, look in the About This Mac dialog in the Apple () menu. Note the screen size, and the hardware model year / time of year, and the version of Mac OS X / macOS. Then check that against the Support document and you will know whether the newer iMac can use the older one as a second screen.

Your two Macs would have to fit these criteria…

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

Hi,

I think there is no way to connect two iMac for dual monitor. You should get secondhand Thunderbolt or LED cinema external display.

Just to add, you might be able to use Duet or Luna Display.