Bimodular Gravity: Unimodularising Bimetric Scalar-Tensor Gravity

It is the object of the present paper to unimodularise a disformal bimetric scalar-tensor theory, thereby defining what we call bimodular gravity. We impose one unimodular constraint per metric via multipliers ${\lambda }_{1,2}$ and show that two natural implementations-a dual fixed-determinant (BUG) and a dual diffeomorphism-invariant (BHT/BDUG) formulation-are classically inequivalent. In BUG the relative volume element $\nu =\sqrt{1-2BX}$ is fixed, enforcing a kinematic constraint on the biscalar and we derive the “bimodular cosmological constant” $\mathrm{\Lambda }={\lambda }_1+\nu {\lambda }_2$. In BHT/BDUG, ${\lambda }_{1,2}$ are individually constant but $\nu$ (hence $BX$) remains dynamical. Recasting the theory in an Einstein-frame form, we derive the biscalar sound speed and identify a subluminal domain $1+B(V+{\lambda }_2)>0$. At the background level, BUG admits constant-roll solutions governed by first-order flow, whereas BHT supports solutions with time-dependent roll. These structural differences yield distinct, in-principle testable predictions for the expansion history, the dark-energy equation of state, and the propagation of biscalar perturbations. Finally, we present a diffeomorphism-invariant completion that correlates the two HT volume forms, reproducing the $\mathrm{\Lambda }$ of BUG on shell whilst maintaining full covariance.