Some Android phones can already be dual booted with (in theory) any other UEFI compatible OS. There’s a whole guide on the PostmarketOS about setting up a dual boot environment.
I briefly tested PostmarketOS on a OnePlus 6T. The core functions seemed fine but overall it lacked functionality, so my plan was to dual boot with LineageOS (a degoogled android project) for the bits that really just want a true android environment to function properly, and PmOS for everything else I could manage. In the end I just wasn’t up for the process of setting up a dual boot, and went with just LineageOS. Been really happy with it so far, and will probably revisit dual booting when PmOS is more feature complete.
Edit, I suppose this doesn’t touch on the idea of running two separate OS’s on separate chips, and it does require a reboot to get the functionality of one OS or another, but besides that hiccup you’d get mostly the same functionality out of less complex hardware.