A two-valley formulation of 1D drift-diffusion transport is presented that takes the coupling between the valleys into account via a new approximation for the non-local electric field. The proposed formulation is suitable for the simulation of III-V heterojunction bipolar transistors as opposed to formulations that employ the single electron gas approximation with a modified velocity-field model, which also causes convergence problems. Based on Boltzmann transport equation simulations, model parameters of the proposed two-valley formulation are given for GaAs, InP, InAs andGaSb at room temperature. Applications of the new formulation are also demonstrated.