Bigravity in tetrad Hamiltonian formalism and matter couplings
  The tetrad approach is used to resolve the matrix square root appearing in the dRGT potential. Constraints and their algebra are derived for the minimal case. It is shown that the number of gravitational degrees of freedom corresponds to one massless and one massive gravitational fields when two sorts of matter separately interact with two metric tensors. The Boulware-Deser ghost is then excluded by two second class constraints. In other case when the matter couples to a linear combination of two tetrads this ghost re-appears. 
Vladimir O. Soloviev
hep-th