Target Display Mode iMac mid 2011 and iMac late 2009

I am trying to get Target Display Mode working between 2 iMacs. I have a mini display cable and am trying to use a 21.5 inch mid-2011 iMac as an extended display for a 27 inch mid-2009 iMac.

When the cable is connected, the 27 inch can enter TDM and extend the display of the 21.5 inch, however it won't work the other way. I want to use the 21.5 inch as the extended display. the 21.5 inch does nothing when I press command + F2 to enter TDM.

One other thing....the 27 is running Yosemite and the 21.5 inch is running Sierra.

Not sure why I can't get the 27 to extend from the 21.5.

Thanks for the help.