I’ve dual booted in the past for testing purposes (to do so you need to share the /boot partition between PureOS and Qubes) but Qubes strongly discourages dual booting.
The reason is that Qubes goes to great lengths to reduce its dom0 attack surface with limited packages and limited functionality because it has full root across all VMs. When you dual boot, you put that at risk if an attacker is able to compromise the less-hardened other OS and use that to also attack Qubes (since they share /boot).