Dual Monitors

Have 2019 iMac, hoping to use 2009 imac for dual monitoring

-have latest update on 2009 for high sierra

-have thunderbolt 3 to mini display cable {that has worked as shared monitor setup)

-have restarted and unplugged multiple times

-have changed keyboard f1,f2 function as suggested

-have pressed command-F2, and command-fn-F2

but still no dual monitor recognition

Any thoughts?