To have grub-mkconfig search for other installed systems and automatically add them to the menu.
First, install os-prober
sudo pacman -S os-prober
Then, mount the drive where the other operating system is located. I did it with thunar.
Run os-prober
sudo os-prober
Finaly, re-run grub-mkconfig
sudo grub-mkconfig -o /boot/grub/grub.cfg