  We consider the effective stress-energy tensors for the foreground and
background sectors in ghost-free bimetric gravity. By considering the
symmetries of the theory, we show that the foreground and background null
energy conditions (NECs) are strongly anti-correlated. In particular, the NECs
can only be simultaneously fulfilled when they saturate, corresponding to
foreground and background cosmological constants. In all other situations,
either the foreground or the background is subject to a NEC-violating
contribution to the total stress-energy.
