I have a 2009 IMAC. I am trying to use it as a dual monitor with my new 2019 IMAC. I purchased the proper cords, thunderbolt three to two adapter. I connected the video port to the proper adapter. Following the above command 2 and cmd fn 2

Use your iMac as a display with Target Display Mode - Apple Support - https://support.apple.com/HT204592 -. HOWEVER, this document is "archived" which means this feature is no longer supported on the new line of Macs and operating system versions that are currently supported by Apple. The document applies only if all system versions to be used date from 2019 or earlier and the display models used are iMacs late 2009-Mid 2014

This could only be done with a very few and very specific older iMacs and support was very limited. Read this topic for a 2021 perspective on this: https://discussions.apple.com/thread/252271436?answerId=254300142022#254300142022

and this 2021 post by Allan Jones about software limitations:

https://discussions.apple.com/thread/252344497?answerId=254428494022#254428494022