How to enable Dual Monitor Setup in Debian 9?

I have a 24" SyncMaster FullHD Monitor on DVI (my main screen) and an older 17" Dell als my secondary Monitor on VGA (because it only has VGA).
My graphics card is a Gigabyte Windforce 2X GTX 960 4GB
I installed Debian 9 with Xfce.

Only the 17" Monitor is recognized, the other one not. But the 24" was connected the whole time. I tried to replug the monitor. When "Configure new displays when connected" was checked (Xfce Display Settings) and when not. Nothing. The monitor runs fine when booting in Windows.

Is that a configuration or a driver issue? How to solve this? Shouldn't the display driver detect and configure the screen automatically?
I havn't tried to install the non-free nvidia drivers yet and I would prefer not to if there's another way.

@biolinguist did you ever fix the issue you had with second displays?

Edit: here's the link to his post @EpicCrumbCake