Q: iMac 5K scale to 1920x1080?

A utility like SwitchResX might be able to come closer, but I haven't tried it yet on a 5K iMac.

Just out of curiosity, why did you need the two screens to have identical pixel counts?

I have confirmed that SwitchResX will allow switching an iMac (Retina 5K, 27-inch, Late 2014) to 1920x1080 resolution.

System Preferences->Displays->OPTION-click scaled which reveals all resolutions http://hints.macworld.com/article.php?story=2013103013020613. Also, http://support.apple.com/kb/HT5891 and http://support.apple.com/kb/PH13814

If none of those do what you want, download the cscreen executable from http://www.pyehouse.com/cscreen/ and use it to reset the resolution to suit your needs. For more details on using cscreen, see http://apple.stackexchange.com/questions/73177/how-to-change-resolution-when-usi ng-screen-sharing-and-no-physical-screen-is-con

27" i7 iMac (Mid 2011) refurb, OS X Yo (10.10.5), Mavs, ML & SL, G4 450 MP w/10.5 & 9.2.2