You are right Paul,

It is in fact working with Windows 10 using Bootcamp which points to a software problem on mac OSX. My two screen are detected as 1920×1200@60Hz + the display of the MacBook Air at 1440×900@60Hz.  Beside the two monitor being recognized, I’m getting a small piece of the screen (around 300px) offseted to the right with a pink vertical line on the U2405fpw.  I tried to connect another monitor on the CSV-5400 (an Acer AL2016W) at Native resolution of 1680×1050@60 but the bandwith seems to be full at this point and the screen is disabled by default.