It's due to the Chromium build being over 12 weeks old and will be fixed on your next apt update && apt dist-upgrade -y cycle (probably in March when the RPF/RPT folks have rebuilt Chromium).
Don't fiddle with the config files.
Run Screen Configuration which has options to configure both screens. (This is basically a front end to xrandr.)
This is a GUI but AFAIK the only way to configure multiple screens.
If you hadn't wasted space with NOOBS you would have plenty of room for Raspbian with Desktop.
You can get the serial number with:
rpi ~$ cat /sys/firmware/devicetree/base/serial-number
So the command to call chromium on the command line with serial number resulted in:
rpi ~$ chromium-browser --incognito --noerrdialogs http://url.com?sn=$(cat /sys/firmware/devicetree/base/serial-number)