Why there are two brightness control file (/sys/class/) in my system

Brightness could be controlled by ACPI or by graphic driver. I suppose the one that controls your brightness is intel_backlight. However, you could easy find it out adding to your /etc/rc.local (before exit 0):

echo 5 > /sys/class/backlight/acpi_video0/brightness

or, alternatively:

echo 5 > /sys/class/backlight/intel_backlight/brightness

It seems that Fn-Keys do change the value of acpi_video0, the problem is that the controlling item is intel_backlight.

If the system starts with the kernel parameter acpi_backlight=vendor, the item acpi_video0 is replaced by the item intel, but then the Fn-Keys can not change the value of this item.