In the past, I've interpreted the phrase "EX hUDATOS KAI PNEUMATOS" as
referring to two "births": natural birth and birth "again/from above".
My reasoning for this was that Jesus seemed to be contrasting spiritual
birth with natural birth in the immediate context.
However, Beasley-Murray in "Baptism in the New Testament" seems to argue
that "EX hUDATOS KAI PNEUMATOS" could not possibly mean that for the
purely grammatical reason that "the unity of the two elements is shown
by the use of the single preposition EK".