  We show that Coulomb drag in hydrodynamic bilayer systems leads to additional
viscosity terms in the hydrodynamic equations, i.e., the drag and drag-Hall
viscosities, besides the well-known kinematic and Hall viscosities. These new
viscosity terms arise from a change of the stress tensor due to the interlayer
Coulomb interactions. All four viscosity terms are tunable by varying the
applied magnetic field and the electron densities in the two layers. At certain
ratios between the electron densities in the two layers, the drag viscosity
dramatically changes the longitudinal transport resulting in a negative drag
conductivity.
