  We show how Newtonian cosmological simulations can be employed to investigate
the non-linear evolution of two particle species in a relativistic context. We
discuss the application for massive neutrinos and other multi-species systems
such as Cold Dark Matter (CDM) plus baryons or Warm Dark Matter (WDM). We
propose a method that allows us to perform simulations including massive
neutrinos and general relativistic effects at almost the same computational
cost as ordinary CDM only N-body simulations, employing tailor-made initial
conditions and a dictionary for the interpretation of the simulation output.
