Two-field disformal transformation and mimetic cosmology
  We extend the disformal transformation to models with two scalar fields and
look at its singular limit. Solving the eigentensor equation for the Jacobian
of the transformation of the metrics we find the two-field extension of the
mimetic scenario in the singular conformal limit. At the background level the
setup mimics the roles of dark matter cosmology. We decompose the perturbations
into the adiabatic and entropy modes in which the adiabatic perturbation is
tangential to the classical trajectory while the entropy mode is perpendicular
to it. We show that the adiabatic mode is frozen while the entropy mode
propagates with the sound speed equal to unity with no instabilities.
