[Haskell-cafe] A small puzzle: inTwain as function of foldr

Bonjour café, A small puzzle: Consider the function inTwain that splits a list of even length evenly into two sublists:

inTwain "Hello world!" ("Hello ","world!")

Is it possible to implement inTwain such that the recursion is done by one of the standard list folds? Is there a general way to tell if a problem can be expressed as a fold? Thank you, Martijn.