  Bimetric gravity can reproduce the accelerated expansion of the Universe,
without a cosmological constant. However, the stability of these solutions to
linear perturbations has been questioned, suggesting exponential growth of
structure in this approximation. We present a simple model of structure
formation, for which an analytical solution is derived. The solution is
well-behaved, showing that there is no physical instability with respect to
these perturbations. The model can yield a growth of structure exhibiting
measurable differences from $\Lambda$CDM.
