How to tether a 2014 MacBook Pro 15" Retina to two Dell Ultra HD 4k 27-Inch Screen LED-Lit Monitors (P2715Q)?
It doesn't appear that your MBP is capable of driving 2 4k displays with the Macbook's primary display working (see Graphics and Video support):

https://support.apple.com/kb/SP704?locale=en_US

Also the OWC Dock isn't capable of driving 2 4k displays so even if your MBP is capable of driving the displays you'll need to use the HDMI port or one of the other Thunderbolt 2 ports to drive the second display:

http://www.owcdigital.com/products/thunderbolt/thunderbolt-2-dock-overview

One thing you may be able to try is to close the Macbook to turn off it's primary display, thus freeing up resources to try to handle both 4k displays.