I have some trouble enabling more than two monitor displays. My T520 is connected to a docking station which has two digital video output, which are in turn connected to two NEC monitors.
For some reason, when I go to Windows | Display Settings, it will not allow me to turn on all three displays at the same time. I can either have a) both monitors on or b) laptop display + 1 monitor.
Whenever I tried to connect to the 3rd display, it will automatically switch off the 2nd display.
My other colleagues with T420 are not having this issue. The T420 & T520 use the same docking station so I don't think that's the issue.
My initial thought is to update the Video Driver which I did but still not working. The T420 has a nVidia card whereas T520 has an on-board Intel one.
The T520 with Nvidia Optimus can support up to four screens, while the T520 with Intel only graphics card can support only two screens.
If you need more screen support, you can consider USB2dvi, eGPU or Vidock.
For more on the various specifications on the T520, please refer to the following resource.