I have two Dell 27" WQHD (2560x1440) monitors and I want to use dual DisplayPort from my port replicator under my T520 (w/ nvidia nvs 4200M graphics [I think]).
My port replicator is a ThinkPad Mini Dock Plus Series 3 (433810U) 90 Watt version.
PROBLEM: I am unable to push more than 1920x1200 to my dual monitors. If I disconnect one of the DisplayPort cables, the monitor still connected switches to 2560x1440, but I am unable to push that resolution to both at the same time.
You can drive two monitors at WQHD or WQXGA on the T520 and T420 by selecting discrete graphics mode in bios menu. Then connect the two external monitors to the displayport (one on the laptop itself and another through the dock).
P.S. When this mode is selected this will disable the optimus mode and therefore you would lose 4 monitors support and can instead only drive two monitors. Also, DVI port on the dock is single link only so it can NOT drive monitors at WQHD or WQXGA resolution (which requires dual link). Only displayport support dual link and WQXGA/WQHD resolution.
If you need dual link dvi out, then you need to purchase active displayport to dual link dvi adapter. However, Lenovo does not produce or market them, so you need to seek alternative retailers for these adapters (they are readily available on the internet eRetailers).