Have you installed the Lenovo OneLink Pro Dock latest firmware and drivers? ThinkPad OneLink Pro Dock - Overview and Service Parts - Lenovo Support (US)
Be sure you are using the Lenovo OneLink Pro Dock software to set the resolution of the displays. See page 7 of the Lenovo OneLink Pro Dock user guide.
Also,, according to the Lenovo OneLink Pro Dock user guide pg 13, these are the resolutions supported via the dock:
Using the software to set the resolution just opens the windows 8.1 screen resolution window so there is nothing special to it.
The manual say DP 2560x1600 but according to the sales documentation it sais "Full Dual video output is supported on ThinkPads with DisplayPort 1.2" which my laptop supports, the resolutions listed in the manual is only if you use both DP and DVI.
When talking to Lenovo support they themself think it should be possible to do but they can't say why it's not working.
I am using the latest firmware and software for the dock.