Dual booting would sacrifice a lot of the hardware-based security feature integration and would be much further from passing attestation checks. GrapheneOS fully supports hardware-based attestation but Google doesn't permit it in the Play Integrity API. Directly booting the fully unmodified stock OS is required to pass the hardware attestation checks for the stock OS. GrapheneOS appears as GrapheneOS in the attestation metadata and a dual boot setup would appear as that specific dual boot setup. Since it would have a bunch of security sacrificed for it, it would be far harder to convince services to permit that. It would be counterproductive.

GrapheneOS has near perfect app compatibility other than the Play Integrity API banning it from the overall tiny number of apps using it. It has per-app compatibility toggles for privacy and security features which trip other anti-tampering checks, find memory corruption bugs in apps, etc. There are a couple known compatibility issues from anti-tampering checks from the secure spawning feature but it has a toggle.

The stock OS isn't what's needed but rather directly booting it from the firmware with 0 modifications. Dual booting would require booting something else and major modifications to deal with hardware APIs not designed for multiple operating systems using them at the same time. Secure element / TEE APIs including the hardware keystore and attestation, etc. are not designed for dual boot. A/B updates, verified boot, firmware updates, etc. would need to be dealt with by the bootloader system. It would be complex and messy. The end result would not be a hardened device or one compatible with standard attestation checks.