We had an interesting experience where our twins were born in CH and we wanted then to have a double barrel surname ie the Surname of the mother followed by a hyphen and the surname of the father. We are both UK nationals.

Under Swiss law because we werent married it is law that the surname of the child/ children take the surname of the mother, but we arent Swiss. Under UK law we call call the kids Mickey Mouse etc if we wanted to.

After a lot of form filling and legal stamps and a couple of months later we received Swiss birth certificates with the double name on, as we had wished.

PS It would have been easier to get married!! But we still havent done so!