dual monitor not working

[Bernat@i2cat]

I have a windows host and a debian guest.
I have enabled the dual monitor mode in configurations (both the number of monitors and the video memory assigned). I have been able to do this with other kind of gests but Debian does not recognize my second monitor (it is never launched btw). If I go to display in the guest it only shows one monitor.

VBoxGuestAdditions installed

Thank you for any clue you could give me!