Are you trying to get 2 duplicate desktops, or one extended desktop across both monitors?
Here's the description of the MacBook graphics capability on Apple's website:
Intel HD Graphics 5300
Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on an external display, both at millions of colors
The only way I could see this working is if you could stretch the 3840 pixels across 2 HD monitors. The problem is I don't think the signal will split over HDMI. In other words, if you bought this docking station with 2 HD monitors and hooked your MacBook up to it, you'd just get 2 duplicate desktops instead of one extended desktop. I think you can only get an extended desktop by using the native retina screen plus one external monitor.
Does that help? I'm no graphics expert, but that's what I'd expect would happen.