Transformer symmetry
Can any winding be used like any other?


My very strong suspicion is that the 1kV isolation rating applies between primary and either of the two secondaries. This should not be taken to mean that 1kV isolation is between the two secondaries. Almost certainly this won't be the case. So, if you are using the transformer as an isolator I would say that "any winding" cannot "be used like any other".