– Install Ubuntu

/boot mount point on a parition (ex: sda1) (>=250 MB)
/ mount point on another partition (ex: sda2)

– Install Qubes Os

Don't use the same boot partition. Leave it installs boot on sda.

– Ubuntu won’t boot.
– Open qubes os, terminal:
– sudo su
– nano /etc/grub.d/40_custom

menuentry "Ubuntu" {
set root=(hd0,2) # boot partition
linux /vmlinuz root=/dev/sda2 quiet splat
initrd /initrd.img
}

– nano /etc/default/grub

...
GRUB_DEFAULT=2 # starts from 0
GRUB_TIMEOUT=10
GRUB_CMDLINE_XEN_DEFAULT="console=none dom0_mem=min:640M dom0_mem=max:1280M iommu=no-igfx ucode=scan smt=off"
...

– grub2-mkconfig -o /boot/grub2/grub.cfg
– grub2-install /dev/sda
– reboot