Setting the rift to extended mode in the Oculus configuration tool results in a constant flicker between system main screen and Oculus Rift at a frequency of .5 sec or so on each screen.
Setting the rift in Direct mode works (and appropriate message in the startup screen appears - This mode is not fully compatible yet); however it is not possible to set the refresh rate for the rift in the Elite configuration options any higher than the main monitor, in this case 1080p@60hz. This result in lagged and jittery game play and induce disorientation fairly quickly.
I7-4790 / AMD R9-290(*2) (15.4b) / 24GB PC1600 / Win8.1 (current) / All drivers to date. Main Monitor (DP -> HDMI) Asus 220p, Rift DK2 (DVI -> HDMI).