a second display. Is this possible?

Only if you revert back to High Sierra on the 2013 iMac and to Catalina on the 2017 iMac. Which is not easy nor wise.

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

iMac models introduced in 2011, 2012, 2013, and mid 2014

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.

The cable you would need is explained in the link den.thed posted.

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

FWIW, there is a possible hardware solution called Luna Display.

https://astropad.com/product/lunadisplay/