i think this will depend if the electrical sources in your network will be considered as separately drived systems or not , for separately drived system , we must use 4 pole circuit breaker to disconnect the neutral of the two sources ( transformer and generators or 2 generators ) to prevent earth fault in the main switchgear.
you can digg the NEC code for the expression ” separately drived systems” and you will find the answer or got to the following site which explain the NEC code by following the link: