A phone that isn't locked to one OS would be fun, but it really depends what OSes you want to support. Apps will be the issue though. So a combination like Android and Sailfish I could see, since you can add android app compatibility to Sailfish. Android plus iOS (as if Apple would ever allow that!) would Facebook the problem that you'd need completely different sets of apps (and duplicate contact books etc).
I currently use Macs for work in part because I got fed up with dual booting (I'd mostly use Linux, but occasionally need apps that weren't available for Linux, and just having to stop everything else to run them was a pain). So it would be important to me that I could access pretty much everything whichever OS I was using.
Of course if you just meant "same hardware, choose your OS" rather than dual boot that is simpler

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