  Neutrino oscillation experiments often employ two identical detectors to
minimize errors due to inadequately known neutrino beam. We examine various
systematics effects related to the prediction of the neutrino spectrum in the
`far' detector on the basis of the spectrum observed at the `near' detector. We
propose a novel method of the derivation of the far detector spectrum. This
method is less sensitive to the details of the understanding of the neutrino
beam line and the hadron production spectra than the usually used `double
ratio' method thus allowing to reduce the systematic errors.
