See playback on external HDMI monitor

I have a 2016 IMac with retina display connected to my late 2009 IMac running in target display mode. I have each display set up as its own space so that I can spread out FCPX Across the two displays as if they were one display. This is working fine.

I would like to connect an HDMI monitor to this system so that FCPX shows the playback viewer on this external monitor. I followed Apple's instructions but they do not work.
I am connecting the monitor to my Thunderbolt port using a Thunderbolt to HDMI cable. As soon as I turn on the external monitor, both my Mac screens dumb down to the external monitors resolution (which is Way lower) and the Macs go into mirroring mode. The external monitor mirrors what I see on the Macs. I see FCPx running in three locations.

I just want the Mac screens to work the way they are with no change and with my external monitor showing only what the final video looks like.

Thanks in advance.

hey there,

i don't use FCPX (yet!)
so i can't be of much help,
but when this behaviour happens to me in other situations,
i go to Display Preferences in System Preferences, and "Turn Mirroring Off"


nick