The MacBook only supports dual displays including the built in display. In other words, it will only support one external monitor. Per the tech specs:
Dual display and video mirroring: simultaneously supports full native resolution on the built-in display and up to 4096-by-2304 resolution at 60Hz on an external display, both at millions of colors
The Lenovo C-Dock doesn't enable your MacBook to supply more display bandwidth than Apple provides. So, the mirroring you are seeing is expected behavior.