Hello Jeroen, the issue that you are experiencing could be related to the version of the graphic driver installed on your system. Even though you said it is the latest version, would you please uninstall it?
Then, restart the system and install the following version. https://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&ProdId=3712&DwnldID=23639&ProductFamily=Graphics&ProductLine=Desktop+graphics+drivers&ProductProduct=3rd+Generation+Intel%C2%AE+Core%E2%84%A2+Processors+with+Intel%C2%AE+HD+Graphics+4000&lang=eng
It could be the same version 3412; however this file is from the graphic list.
Already tried this driver. I now found out that the only way I can get it to work is if I use two HDMI-DVI cables. Then I have no problem using two monitors.
The driver only then recognizes both monitors as a "Digital Display", otherwise the monitors are marked as "Digital Television" in de Display Manager.