Well, dual boot has been done with Windows Mobile and Android and also between Maemo 5 and Android on N900. Don't think there are any phones which can dual boot Android and Symbian. There was this claim from a guy running Android on N95, but not sure how true it is. Android is not capable of running on the kind of minimal hardware that Symbian runs on. In any case, all these attempts are more like proof of concept than a practical solution.