Having Problem Dual Screening Late 2009 iMac to Late 2015 iMac
I have read tried and failed to dual screen my 2 iMacs
In the late 2009 I have an mini display port to HDMI cable (male)
In the late 2015 I have a thunderbolt 2 to HDMI cable (female)
Command F2 does not seem to work on either screen
Says: 27" 2009 iMac.
If the above 2 are true, I would use a miniDisplayPort-to-miniDisplayPort cable between the 2 Macs. A Thunderbolt 2 port is also a miniDisplayPort, so you just plug directly into the miniDisplayPort on the 2009 and the TB2 port on the 2015.
When I had a 27" 2009 iMac I successfully used Target Display Mode using that cable from the 2009 to a 2011 15" Macbook Pro with Thunderbolt 2 ports. So I know that will work.
In my opinion you are using way too many adapters for your approach. Just a simple mDP-to-mDP cable should be all you need.
Ok - re-boot back High Sierra - if that is possible and change the cable - many tx