I am using my iMac 2017 5k as my main machine and want to use my 21 inch 2017 iMac as a second screen. I have learnt from the previous question that I asked that the target display mode is not going to work. What are some workarounds to use the real estate on the second 21 inch 2017 iMac while on the main machine?
There's a product called AirDisplay from Avatron Software that allows you to extend your display to another Mac.
Extend your main Mac screen to a nearby iOS or Android device or to another Mac or PC. Increase your productivity with an extra screen or mirror your main display to up to 4 screens.