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Seems impossible to me, because you want to host it at two different locations/hosters. The dns directs your domain name to one ip adres, so it is only possible when the master dns redirects to your own dns server which can have a rule: if ip 1 = unavaible then redirect to ip 2.
A better solution would be (if you have the money) to find one place that mirrors your site on two different ip's.