I have two iMacs. How can I use one as a secondary display screen for another?

Please carefully read Target Display Mode (Target Display Mode), it might be possible however Target Display Mode has been dead about 6 years. Please remember that Macs are computers, they are not displays for other devices.

There are two third party apps that will allow you use another mac as a secondary display. Duet Display and Luna Display. Depending on your performance needs they may work for you.

I hope this helps.

Paul