Use a Late 2009 iMac Running High Sierra as a Second Monitor?

I just bought 2019 27" iMac running Catalina and was wondering if I can use my late 2009 iMac running High Sierra as a second monitor. If so, how? Thanks MPUers!

Here you go:

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You’re my hero, seriously when I did a search this did not come up. Thanks again!

By the way, I’m actually running a 2009 iMac as my main machine with High Sierra as my MacOS.

  • Upgraded the HDD to a 2 TB SSD
  • Maxed out Ram to 16 GB

I saw several articles when I searched but they all said the machine needed to be 2011 or later (I did not read the article you linked to), so I’m curious to hear how this works out.